CHAPTER VII
THE PROSOMATIC SEGMENTS OF LIMULUS AND ITS ALLIES
Comparison of the trigeminal with the prosomatic region.—The prosomatic appendages of the Gigantostraca.—Their number and nature.—Endognaths and ectognath.—The metastoma.—The coxal glands.—Prosomatic region of Eurypterus compared with that of Ammocœtes.—Prosomatic segmentation shown by muscular markings on carapace.—Evidence of cœlomic cavities in Limulus.—Summary.
The derivation of the olfactory organs of the vertebrate from the olfactory antennæ of the arthropod in the last chapter is confirmatory proof of the soundness of the proposition put forward in Chapter IV., that the segmentation in the cranial region of the vertebrate was derived from that of the prosomatic and mesosomatic regions of the palæostracan ancestor. Such a segmentation implies a definite series of body-segments, corresponding to the mesomeric segmentation of the vertebrate, and a definite series of appendages corresponding to the splanchnic segmentation of the vertebrate.
Of the foremost segments belonging to the supra-œsophageal region characterized by the presence of the median eyes, of the lateral eyes, and of the olfactory organs, a wonderfully exact replica has been shown to exist in the pineal eyes, the lateral eyes, and the olfactory organ of the vertebrate, belonging, as they all do, to the supra-infundibular region.
Of the infra-œsophageal segments belonging to the prosoma and mesosoma respectively, the correspondence between the mesosomatic segments carrying the branchial appendages and the uterus, with those in the vertebrate carrying the branchiæ and the thyroid gland respectively, has been fully proved in previous chapters.
There remain, then, only the segments of the prosomatic region to be considered, a region which, both in the vertebrate and invertebrate, is never respiratory in function but always masticatory, such mastication being performed in Limulus and its allies by the muscles which move the foot-jaws or gnathites, which are portions of the prosomatic appendages specially modified for that purpose, and in the vertebrates by the masticatory muscles, which are always innervated by the trigeminal or Vth cranial nerve. This comparison implies that the motor part of the trigeminal nerve originally supplied the prosomatic appendages.
The investigations of van Wijhe and of all observers since the publication of his paper prove that in this trigeminal region, as in the vagus region, a double segmentation exists, of which the ventral or splanchnic segments, corresponding to the appendages in the invertebrate, are supplied by the trigeminal nerves, while the dorsal or somatic segments, corresponding to the somatic segments in the invertebrate, are supplied by the IIIrd or oculomotor and the IVth or trochlear nerves—nerves which supply muscles moving the lateral eyes.
In accordance, then, with the evidence afforded by the nerves of the branchial segments, it follows that the muscles supplied by the motor part of the trigeminal ought originally to have moved the appendages belonging to a series of prosomatic segments. On the other hand, the eye-muscles ought to have belonged to the body-part of the prosomatic segments, and must therefore have been grouped originally in a segmental series corresponding to the prosomatic appendages.
The evidence for and against this conclusion will be the subject of consideration in this and the succeeding chapters. At the outset it is evident that any such comparison necessitates an accurate knowledge of the number of the prosomatic segments in the Gigantostraca and of the nature of the corresponding appendages.
In all this group of animals, the evidence as to the number of segments in either the prosomatic or mesosomatic regions is given by—
1. The number of appendages.
2. The segmental arrangement of the muscles of the prosoma or mesosoma respectively.
3. The segmental arrangement of the cœlomic or head-cavities.
4. The divisions of the central nervous system, or neuromeres, together with their outgoing segmental nerves.
It follows, therefore, that if from any cause the appendages are not apparent, as is the case in many fossil remains, or have dwindled away and become insignificant, we still have the muscular, cœlomic, and nervous arrangements left to us as evidence of segmentation in these animals, just as in vertebrates.
In this prosomatic region, we find in Limulus the same tripartite division of the nerves as in the mesosomatic region, so that the nerves to each segment may be classed as (1) appendage-nerve; (2) sensory or dorsal somatic nerve, supplying the prosomatic carapace; (3) motor or ventral somatic nerve, supplying the muscles of the prosoma, and containing possibly some sensory fibres. The main difference between these two regions in Limulus consists in the closer aggregation of the prosomatic nerves, corresponding to the concentration of the separate ganglia of origin in the prosomatic region of the brain.
The number of prosomatic segments in Limulus is not evident by examination of the prosomatic carapace, so that the most reliable guide to the segmentation of this region is given by the appendages, of which one pair corresponds to each prosomatic segment.
The number of such segments, according to present opinion, is seven, viz.:—
(1) The foremost segment, which bears the cheliceræ.
(2, 3, 4, 5, 6) The next five segments, which carry the paired locomotor appendages; and
(7) The last segment, to which belongs a small abortive pair of appendages, known by the name of the chilaria, situated between the last pair of locomotor appendages and the operculum or first pair of mesosomatic appendages. These appendages are numbered from 1-7 in the accompanying drawing (Fig. 103).
Of these seven pairs of appendages, the significance of the first and the last has been matter of dispute. With respect to the first pair, or the cheliceræ, the question has arisen whether their nerves belong to the infra-œsophageal group, or are in reality supra-œsophageal.
It is instructive to observe the nature and the anterior position of this pair of appendages in the allied sea-scorpions, especially in Pterygotus, where the only chelate organs are found in these long, antennæ-like cheliceræ. In Slimonia and in Stylonurus they are supposed by Woodward to be represented by the small non-chelate antennæ seen in Fig. 8, B and C (p. 27), taken from Woodward. If such is the case, then these figures show that a pair of appendages is missing in each of these forms, for they possess only five free prosomatic appendages instead of six, as in Limulus and in Pterygotus. Similarly, Woodward only allowed five appendages for Pterygotus, so that his restorations were throughout consistent. Schmidt, in Pterygotus osiliensis has shown that the true number was six, not five, as seen in his restoration given in Fig. 8, A (p. 27).
Fig. 103.—Ventral Surface of Limulus. (Taken from Kishinouye.)
The gnathic bases of the appendages have been separated from those of the other side to show the promesosternite or endostoma (End.).
With respect to Eurypterus, Schmidt figures an exceedingly minute pair of antennæ between the coxal joints of the first pair of appendages, thus making six pairs of appendages. Gerhard Holm, however, in his recent beautiful preparations from Schmidt's specimens and others collected at Rootziküll, has proved most conclusively that the cheliceræ of Eurypterus were of the same kind as those of Limulus. I reproduce his figure (Fig. 104) showing the small chelate cheliceræ (1) overhanging the mouth orifice, just as in Limulus or in Scorpio.
So, also, since Woodward's monograph, Laurie has discovered in Slimonia acuminata a small median pair of chelate appendages exactly corresponding to the cheliceræ of Limulus, or of Eurypterus, or of Scorpio. We may, therefore, take it for granted that such was also the case in Stylonurus, and that the foremost pair of prosomatic appendages in all these extinct sea-scorpions were in the same position and of the same character as the cheliceræ of the scorpions.
Fig. 104.—Eurypterus Fischeri. (From Holm.)
In the living scorpion and in Limulus the nerves to this pair of appendages undoubtedly arise from the foremost prosomatic ganglia, and the reason why they appear to belong to the supra-œsophageal brain-mass has been made clear by Brauer's investigations on the embryology of Scorpio; for he has shown that the cheliceral ganglia shift from the ventral to the dorsal side of the œsophagus during development, thus becoming pseudo-supra-œsophageal, though in reality belonging to the infra-œsophageal ganglia. This cheliceral pair of appendages is, in all probability, homologous with the second pair of antennæ in the crustacea.
I conclude, then, that the cheliceræ must truly be included in the prosomatic group, but that they stand in a somewhat different category to the rest of the prosomatic appendages, inasmuch as they take up a very median anterior and somewhat dorsal position, and their ganglia of origin are also exceptional in position.
Next for consideration come the chilaria (7 in Fig. 103), which Lankester did not consider to belong to appendages at all, but to be a peculiar pair of sternites. Yet their very appearance, with their spinous hairs corresponding to those of the other gnathites and their separate nerve-supply, all point distinctly to their being a modified pair of appendages, and, indeed, the matter has been placed beyond doubt by the observations of Kishinouye, who has found embryologically that they arise in the same way as the rest of the prosomatic appendages, and belong to a distinct prosomatic segment, viz. the seventh segment. In accordance with this, Brauer has found that in the scorpion there is in the embryo a segment, whose appendages degenerate, which is situated between the segment bearing the last pair of thoracic appendages and the genital operculum—a segment, therefore, comparable in position to the chilarial segment of Limulus.
Coming now to the five locomotor appendages, we find that they resemble each other to a considerable extent in most cases, with, however, certain striking differences. Thus in Limulus they are chelate, with their basal joints formed as gnathites, except in the case of the fifth appendage, in which the extremity is modified for the purpose of digging in the sand. In Pterygotus, Slimonia, Eurypterus, the first four of these appendages are very similar, and are called by Huxley and Woodward endognaths; in all cases they possess a basal part or sterno-coxal process, which acts as a gnathite or foot-jaw, and a non-chelate tactile part, which possesses no prehensile power, and in most cases could have had no appreciable share in locomotion, called by Huxley and Woodward the palpus. These small palps were probably retractile, and capable of being withdrawn entirely under the hood. The fifth appendage is usually different, being a large swimming organ in Pterygotus, Eurypterus, and Slimonia (Figs. 8 and 104), and is known as the ectognath.
Finally, in Drepanopterus Bembycoides, as stated by Laurie, all five locomotor appendages are built up after the same fashion, the last one not being formed as a paddle-shaped organ or elongated as in Stylonurus, but all five possess no special locomotor or prehensile power. According to Laurie this is a specially primitive form of the group.
It is significant to notice from this sketch that with the absence of special prehensile terminations such as chelæ, or the absence of special locomotor functions such as walking or swimming, these appendages tend to dwindle and become insignificant, taking up the position of mere feelers round the mouth, and at the same time are concentrated and pressed closely together, so that their appendage-nerves must also be close together.
This sketch therefore shows us that—
Of the six foremost prosomatic appendages, the cheliceræ and the four endognaths were, at the time when the vertebrates first appeared, in very many cases dwindling away; the latter especially no longer functioned as locomotor appendages, but were becoming more and more mere palps or tentacles situated round the mouth, which could by no possibility afford any help to locomotion.
On the contrary, the sixth pair of appendages—the ectognaths—remained powerful, being modified in many cases into large oar-like limbs by which the animal propelled itself through the water.
It is a striking coincidence that those ancient fishes, Ptericthys and Bothriolepis, should have possessed a pair of large oar-like appendages.
At this time, then, in strong contrast to the endognaths, the ectognaths, or sixth pair of appendages, remained strong and vigorous. What about the seventh pair, the chilaria of Limulus?
Of all the prosomatic appendages these are the most interesting from the point of view of my theory, for whereas in the scorpion of the present day they have dwindled away and left no trace except in the embryo, in the sea-scorpions of old, far from dwindling, they had developed and become a much more important organ than the chilaria of Limulus.
In all these animals a peculiarly striking and unique structure is found in this region known by the name of the metastoma, or lip-plate (Figs. 8 and 104 (7)); it is universally considered to be formed by the fusion of the two chilarial appendages.
All observers are agreed that this lip-plate was freely movable. Nieskowski considers that the movement of the metastoma was entirely in a vertical direction, whereby the cleft which is seen between the basal joints of all the pairs of locomotor appendages could be closed from behind. Woodward says it no doubt represents the labium, and served more effectually to enclose the posterior part of the buccal orifice, being found exteriorly to the toothed edges of the ectognaths or maxillipedes. Schmidt agrees with Nieskowski, and looks on the mestasoma as forming a lower lip within which the bases of the ectognaths worked.
Fig. 105.—Diagram of Sagittal Median Section through A, Limulus, B, Eurypterus.
Quite recently Gerhard Holm has worked over again the very numerous specimens of Eurypterus Fischeri, which are obtainable at Rootziküll, and has thrown new light on the relation of the metastoma to the mouth-parts. His preparations show clearly that the true lower lip of Eurypterus was not the metastoma, for when the metastoma is removed another plate (End., Fig. 105, B) situated internally to it is disclosed, which, in his view, corresponds to the sternite between the bases of the pro-somatic appendages in Limulus, i.e. to the sternite called by Lankester, the pro-mesosternite (End., Fig. 103). This inner plate formed with the metastoma ((7) Fig. 105) and the ectognaths (6) a chamber closed posteriorly, within which the bases of the ectognaths worked. In other words, the removal of the metastoma discloses in Eurypterus the true anterior ventral surface of the animal which corresponds to that of Limulus, or of the scorpion group, with its pro-mesosternite and laterally attached gnathites or sterno-coxal processes. To this inner plate or pro-mesosternite Holm gives the name of endostoma.
To the anterior edge of the endostoma a thinner membrane is attached which passes inwards in the direction of the throat, and forms, therefore, the lower lip (Hyp., Fig. 105, B) of the passage of the mouth (olf. p.). This membrane bears upon its surface a tuft of hairs, which he thought were probably olfactory in function. Consequently, in his preliminary communication, he describes this lower lip as forming, in all probability, an olfactory organ; in his full communication he repudiates this suggestion, because he thinks it unlikely that such an organ would be situated within the mouth. I feel sure that if Holm had referred to Croneberg's paper, and seen how the true mouth in all the scorpion group is situated at the base of an olfactory passage, he would have recognized that his first suggestion is in striking accordance with the nature of the entrance to the mouth in other scorpions.
That Eurypterus also possessed a camerostome (cam.) seems to follow of necessity from its evident affinities both with Limulus and the scorpions. We see, in fact, that the mouth of these old sea-scorpions was formed after the fashion of Limulus, surrounded by masticatory organs in the shape of foot-jaws, and yet foreshadowed that of the scorpion, so that an ideal sagittal section of one of these old palæostracan forms would be obtained by the combination of actual sagittal sections through Limulus and a member of the scorpion group, with, at the same time, a due recognition of Holm's researches. Such a section is represented in Fig. 105, B, in which I have drawn the central nervous system and its nerves, the median eyes (C.E.), the olfactory organs (Cam.), the pharynx (Ph.), œsophagus (œs.), and alimentary canal (Al.), but have not tried to indicate the lateral eyes. I have represented the prosomatic appendages by numbers (1-7), and the foremost mesosomatic segments by numbers (8-13). I have placed the four endognaths and the nerves going to them close together, and made them small, mere tentacles, in recognition of the character of these appendages in Eurypterus, and have indicated the position and size of the large ectognath, with its separate nerve, by (6). If among the ancient Eurypterus-like forms, which were living at the time when vertebrates first appeared, there were some in which the ectognaths also had dwindled to a pair of tentacles, then such animals would possess a prosomatic chamber formed by a metastoma or accessory lip, within which were situated five pairs of short tactile appendages or tentacles. If the vertebrate were derived from such an animal, then the trigeminal nerve, as the representative of these prosomatic appendage-nerves, ought to be found to supply the muscles of this accessory lip and of these five pairs of tentacles in the lowest vertebrate.
This prosomatic or oral chamber, as it might be called, was limited posteriorly by the fused metastoma (7) and operculum (8), so that if in the same imaginary animal one imagines that the gill-chambers, instead of being separate, are united to form one large respiratory chamber, then, in such an animal, a prosomatic oral chamber, in which the prosomatic appendages worked, would be separated from a mesosomatic respiratory chamber by a septum composed of the conjoined basal portions of the mesosomatic operculum and the prosomatic metastoma, as indicated in the diagram. In this septum the nerves to the last prosomatic appendage (equivalent to the last part of the trigeminal in the vertebrate) and to the first mesosomatic (equivalent to the thyroid part of the facial) would run, as shown in the figure, close together in the first part of their course, and would separate when the ventral surface was reached, to pass headwards and tailwards respectively.
The Coxal Glands.
One more characteristic of these appendages requires mention, and that is the excretory glands situated at the base of the four endognaths known as the coxal glands. These glands are the main excretory organs in Limulus and the scorpions, and extend into the basal segments or coxæ of the four endognaths, not into those of the ectognaths or the chilaria (or metastoma). Hence their name, coxal glands; and, seeing the importance of the excretory function, it is likely enough that they would remain, even when the appendages themselves had dwindled away. With the concentration and dwindling of the endognaths these coxal glands would also be concentrated, so that in the diagram (Fig. 105) they would rightly be grouped together in the position indicated (cox. gl.).
Such a diagram indicates the position of all the important organs of the head-region except the special organs for taste and hearing. These, for the sake of convenience, I propose to take separately, in order at this stage of my argument not to overburden the simplicity of the comparison I desire to make with too much unavoidable detail.
The Prosomatic Region of Ammocœtes.
Let us now compare this diagram with that of the corresponding region in Ammocœtes and see whether or no any points of similarity exist.
With respect to this region, as in so many other instances already mentioned, Ammocœtes occupies an almost unique position among vertebrates, for the region supplied by the trigeminal nerve—the prosomatic region—consists of a large oral chamber which was separated from the respiratory chamber in the very young stage by a septum which is subsequently broken through, and so the two chambers communicate.
This chamber is bounded by the lower lip ventrally, the upper lip and trabecular region dorsally, and the remains of the septum or velum laterally and posteriorly. It contains a number of tentacles arranged in pairs within the chamber so as to form a sieve-like fringe inside the circular mouth; of these, the ventral pair are large, fused together, and attached to the lower lip.
All the muscles belonging to this oral chamber are of the visceral type, and are innervated by the trigeminal nerve. In accordance with the evidence obtained up to this point this means that such an oral chamber was formed by the prosomatic appendages of the invertebrate ancestor, similarly to the oral chamber just figured for Eurypterus.
This chamber in the full-grown Ammocœtes is not only open to the respiratory chamber, but is bounded by the large upper lip (U.L., Fig. 106, D). On the dorsal surface of this region, in front of the pineal eye (C.E.), is the most conspicuous opening of the olfactory tube (Na.), which olfactory tube passes from the dorsal region to the ventral side to terminate blindly at the very spot where the infundibulum comes to the surface of the brain. Here, also, is situated that extraordinary glandular organ known as the pituitary body (Pit.). A sagittal section, then, in diagram form, of the position of parts in the full-grown Ammocœtes, would be represented as in Fig. 106, D.
But, as argued out in the last chapter, the diagram of the adult Ammocœtes must be compared with that of a cephalaspidian fish; the diagram of the palæostracan must be compared with the larval condition of Ammocœtes. In other words, Fig. 106, B, must be compared with Fig. 106, C, which represents a section through the larval Ammocœtes as it would appear if it reached the adult condition without any forward growth of the upper lip or any breaking through of the septum between the oral and respiratory chambers. The striking similarity between this diagram and that of Eurypterus becomes immediately manifest even to the smallest details. The only difference between the two, except, of course, the notochord, consists in the closure of the mouth opening (o), in Fig. 106, B, by which the olfactory passage (olf. p.) of the scorpion becomes converted into the hypophysial tube (Hy.), Fig. 106, C, and later into the nasal tube (Na.), Fig. 106, D, of the full-grown Ammocœtes. That single closure of the old mouth is absolutely all that is required to convert the Eurypterus diagram into the Ammocœtes diagram.
Such a comparison immediately explains in the simplest manner a number of anatomical peculiarities which have hitherto been among the great mysteries of the vertebrate organization. For not only do the median eyes (C.E.) correspond in position in the two diagrams, and the infundibular tube (Inf.) and the ventricles of the brain (C.C.) correspond to the œsophagus (œs.) and the cephalic stomach (Al.), as already fully discussed; but even in the very place where the narrow œsophagus opened into the wider chamber of the pharynx (Ph.), there, in all the lower vertebrates, the narrow infundibular tube opens into the wider chamber of the membranous saccus vasculosus (sac. vasc.). This is the last portion of the membranous part of the tube of the central nervous system which has not received explanation in the previous chapters, and now it is seen how simple its explanation is, how natural its presence—it represents the old pharyngeal chamber of the palæostracan ancestor.
Fig. 106.—Diagram of Sagittal Median Section through B, Eurypterus; C, Larval Ammocœtes; D, Full-grown Ammocœtes.
Next among the mysteries requiring explanation is the pituitary body, that strange glandular organ always found so closely attached to the brain in the infundibular region that when it is detached in taking out the brain it leaves the infundibular canal patent right into the IIIrd ventricle. A comparison of the two diagrams indicates that such a glandular organ (Pit.), Fig. 106, C, was there because the coxal excretory glands (cox. gl.), Fig. 106, B, were in a similar position in the palæostracan ancestor—that, indeed, the pituitary body is the descendant of the coxal glands.
Finally, the diagrams not only indicate how the mesosomatic appendage-nerves supplying in the one case the operculum and the respiratory appendages correspond to the respiratory group of nerves, VII., IX., X., supplying in the other case the thyroid, hyoid, and branchial segments, but also that a similar correspondence exists between the prosomatic appendage-nerves in the one case and the trigeminal nerve in the other; a correspondence which supplies the reason why in the vertebrate a septum originally existed between an oral and respiratory chamber.
Such a comparison, then, leads directly to the suggestion that the trigeminal nerve originally supplied the prosomatic appendages, such appendages being: 1. The metastoma, which has become in Ammocœtes the lower lip supplied by the velar or mandibular branch of the trigeminal nerve (7); 2. The ectognath, which has become the large median ventral tentacle, called by Rathke the tongue, supplied by the tongue nerve (6); 3. The endognaths, which have been reduced to tentacles and are supplied by the tentacular branch of the trigeminal nerve (2, 3, 4, 5).
I have purposely put these two diagrams of the larval Ammocœtes and of Eurypterus before the minds of my readers at this early stage of my argument, so as to make what follows more understandable. I propose now to consider fully each one of these suggestive comparisons, and to see whether or no they are in accordance with the results of modern research.
In the first instance, the diagrams suggest that the trigeminal nerve originally supplied the prosomatic appendages of the palæostracan ancestor, while the eye-muscle nerves supplied the body-muscles of the prosoma.
As these appendages did not carry any vital organs such as branchiæ, but were mainly locomotor and masticatory in function, it follows that their disappearance as such would be much more complete than that of the mesosomatic branchial appendages. Most probably, then, in the higher vertebrates no trace of such appendages might be left; consequently the segmentation due to their presence would be very obscure, so that in this region the very reverse of what is found in the region of the vagus nerve would be the rule. There branchiomeric segmentation is especially evident, owing to the persistence of the branchial part of the branchial appendages; here, owing to the disappearance of the appendages, the segmentation is no longer branchiomeric, but essentially mesomeric in consequence of the persistence of the somatic eye-muscles.
In addition to the evidence of the appendages themselves, the number of prosomatic segments is well marked out in all the members of the scorpion group by the divisions of the central nervous system into well-defined neuromeres in accordance with the appendages, a segmentation the reminiscence of which may still persist after the appendages themselves have dwindled or disappeared. In accordance with this possibility we see that one of the most recent discoveries in favour of a number of segments in the head-region of the vertebrate is the discovery in the early embryo of a number of partial divisions in the brain-mass, forming a system of cephalic neuromeres which may well be the rudiments of the well-defined cephalic neuromeres of animals such as the scorpion.
The Evidence of the Prosomatic Musculature.
Even if the appendages as such become obscure, yet their muscles might remain and show evidence of their presence. The most persistent of all the appendage-muscles are the basal muscles which pass from coxa to carapace and are known by the name of tergo-coxal muscles. They are large, well marked, segmentally arranged muscles, dorso-ventral in direction, and, owing to their connecting the limb with the carapace, are likely to be retained even if the appendage dwindles away.
The muscular system of Limulus and Scorpio has been investigated by Benham and Miss Beck under Lankester's direction, and the conclusions to which Lankester comes are these—
The simple musculature of the primitive animal from which both Limulus and the scorpions arose consisted of—
1. A series of paired longitudinal dorsal muscles passing from tergite to tergite of each successive segment.
2. A similar series of paired longitudinal ventral muscles.
3. A pair of dorso-ventral muscles passing from tergite to sternite in each segment.
4. A set of dorso-ventral muscles moving the coxa of each limb in its socket.
5. A pair of veno-pericardial muscles in each segment.
Of these groups of muscles, any one of which would indicate the number of segments, Groups 1 and 2 do not extend into the prosomatic region, and Group 5 extends only as far as the heart extends in the case of both Limulus and the Scorpion group; so that we may safely conclude that in the Palæostraca the evidence of somatic segmentation in the prosomatic region would be given, as far as the musculature is concerned, by the dorso-ventral somatic muscles (Group 3), and of segmentation due to the appendages by the dorso-ventral appendage musculature (Group 4).
Therefore, if, as the evidence so far indicates, the vertebrate has arisen from a palæostracan stock, we should expect to find that the musculature of the somatic segments in the region of the trigeminal nerve did not resemble the segmental muscles of the spinal region, was not, therefore, the continuation of the longitudinal musculature of the body, but was dorso-ventral in position, and that the musculature of the splanchic segments resembled that of the vagus region, where, as pointed out in Chapter IV., the respiratory muscles arose from the dorso-ventral muscles of the mesosomatic appendages. This is, of course, exactly what is found for the muscles which move the lateral eyes of the vertebrate; these muscles, innervated by the IIIrd, IVth, and VIth nerves, afford one of the main evidences of segmentation in this region, are always grouped in line with the somatic muscles of spinal segments, and yet cannot be classed as longitudinal muscles. They are dorso-ventral in direction, and yet belong to the somatic system; they are exactly what one ought to find if they represent Group 3—the dorso-ventral body-muscles of the prosomatic segments of the invertebrate ancestor.
The interpretation of these muscles will be given immediately; at present I want to pass in review all the different kinds of evidence of segmentation in this region afforded by the examination of the invertebrate, whether living or fossil, so as to see what clues are left if the evidence of appendages fails us. I will take in the first instance the evidence of segmentation afforded by the presence of the musculature of Group 4, even when, as in the case of many fossils, no appendages have yet been found. In such animals as Mygale and Phrynus the prosomatic carapace is seen to be marked out into a series of elevations and depressions, and upon removing the carapace we see that these elevations correspond with and are due to the large tergo-coxal muscles of the appendages; so that if such carapace alone were found fossilized we could say with certainty: this animal possessed prosomatic appendages the number of which can be guessed with more or less certainty by these indications of segments on the carapace.
In those forms, then, which are only known to us in the fossil condition, in which no prosomatic appendages have been found, but which possess, more or less clearly, radial markings on the prosomatic carapace resembling those of Phrynus or Mygale, such radial markings may be interpreted as due to the presence of prosomatic appendages, which are either entirely concealed by the prosomatic carapace or dorsal head-plate, or were of such a nature as not to have been capable of fossilization.
The group of animals in question forms the great group of animals, chiefly extinct, classified by H. Woodward under the order of Merostomata. They are divided by him into the sub-order of Eurypteridæ, which includes—(1) Pterygotus, (2) Slimonia, (3) Stylonurus, (4) Eurypterus, (5) Adelophthalmus, (6) Bunodes, (7) Arthropleura, (8) Hemiaspis, (9) Exapinurus, (10) Pseudoniscus; and the sub-order Xiphosura, which includes—(1) Belinurus, (2) Prestwichia, (3) Limulus.
Fig. 107.—Phrynus Margine-Maculata.
Ce., median eyes; le., lateral eyes; glab., median plate over brain; Fo., fovea.
Fig. 108.—Phrynus sp. (?). Carapace removed.
cam., camerostome; pl., plastron.
The evidence of the Xiphosura and of the Hemiaspidæ conclusively shows, in Woodward's opinion, that the Merostomata are closely related to the Trilobita, and the Hemiaspidæ especially are supposed to be intermediate between the trilobites and the king-crabs. They are characterized, as also Belinurus and Prestwichia, by the absence of any prosomatic appendages, so that in these cases, as is seen in Fig. 12 (p. 30), representing Bunodes lunula, found in the Eurypterus layer at Rootziküll, we have an animal somewhat resembling Limulus in which the prosomatic appendages have either dwindled away and are completely hidden by the prosomatic carapace, or became so soft as not to be preserved in the fossilized condition. The appearance of the prosomatic carapace is, to my mind, suggestive of the presence of such appendages, for it is marked out radially, as is seen in the figure, in a manner resembling somewhat the markings on the prosomatic carapace of Mygale or Phrynus; the latter markings, as already mentioned, are due to the aponeuroses between the tergo-coxal muscles of the prosomatic appendages which lie underneath and are attached to the carapace.
A very similar radial marking is shown by Woodward in his picture of Hemiaspis limuloides, reproduced in Fig. 109, found in the Lower Ludlow beds at Leintwardine. This species has yielded the most perfect specimens of the genus Hemiaspis, which is recognized as differing from Bunodes by the possession of a telson.
It is striking to find that similar indications of segments have been found on the dorsal surface of the head-region in many of the most ancient extinct fishes, as will be fully discussed later on.
Fig.109.—Hemiaspis limuloides. (From Woodward.)
gl., glabellum.
The Evidence of Cœlomic Cavities.
In the head-region of the vertebrate, morphologists depend largely upon the embryonic divisions of the mesoderm for the estimation of the number of segments, and, therefore, upon the number of cœlomic cavities in this region, the walls of which give origin to the striated muscles of the head, so that the question of the number of segments depends very largely upon the origin of the muscles from the walls of these head-cavities. It is therefore interesting to examine whether a similar criterion of segmentation holds good in such a segmented animal as Limulus, or in the members of the scorpion group, in which the number of segments are known definitely by the presence of the appendages. In Limulus we know, from the observations of Kishinouye, that a series of cœlomic cavities are formed embryologically in the various segments of the mesosoma and prosoma, in a manner exceedingly similar to their mode of formation in the head-region of the vertebrate, and he has shown that in the mesosoma a separate cœlomic cavity exists for each segment, so that just as the dorso-ventral somatic muscles are regularly segmentally arranged in this region, so are the cœlomic cavities, and we should be right in our estimation of the number of segments in this region by the consideration of the numerical correspondence of these cavities with the mesomatic appendages. Similarly, in the vertebrate, we find every reason to believe that a single, separate head-cavity corresponds to each of the branchial segments in the opisthotic region, and therefore we should estimate rightly the number of segments by the division of the mesoderm in this region.
In the prosomatic region of Limulus, the dorso-ventral muscles are not arranged with such absolute segmental regularity as in the mesosomatic region, and Kishinouye's observations show that the cœlomic cavities in this region do not correspond absolutely with the number of prosomatic appendages. His words are:—
A pair of cœlomic cavities appears in every segment except the segments of the 2nd, 3rd, and 4th appendages, in which cœlomic cavities do not appear at all. At least eleven pairs of these cavities are produced. The eleventh pair belongs to the seventh abdominal segment.
The first pair of cœlomic cavities is common to the cephalic lobe and the segment of the first appendage (i.e. the cheliceræ).
The second cœlomic cavity belongs to the segment of the fifth appendage. It is well developed.
The ventral portion of the second cœlomic cavity remains as the coxal gland.
Consequently, if we were to estimate the number of segments in this region by the number of cœlomic cavities we should not judge rightly, for we should find only four cavities and seven appendages, as is seen in the following table:—
| LIMULUS. | VERTEBRATE. | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Segments. | Appendages. | Eurypterid appendages. | Cœlomic cavities. |
Cœlomic cavities. | ||||||
| Prosomatic | 1 | Cheliceræ or 1st locomotor. | Cheliceræ | 1 | Anterior | |||||
| 2 | 2nd locomotor | Endognaths | 2 | Premandibular | ||||||
| 3 | 3rd " | |||||||||
| 4 | 4th " | |||||||||
| 5 | 5th " | |||||||||
| 6 | 6th " | Ectognath | 3 | Mandibular | ||||||
| 7 | Chilaria | Metastoma | 4 | |||||||
| Mesosomatic | 8 | Operculum | Operculum | Genital | 5 | Hyoid | ||||
| 9 | 1st branchial | 1st branchial | 6 | |||||||
| 10 | 2nd " | 2nd branchial | 7 | 1st branchial | ||||||
| 11 | 3rd " | 3rd " | 8 | 2nd " | ||||||
| 12 | 4th " | 4th " | 9 | 3rd " | ||||||
| 13 | 5th " | 5th " | 10 | 4th " | ||||||
| 14 | 6th " | 11 | ||||||||
The second cavity would in reality represent four segments belonging to the 2nd, 3rd, 4th, 5th locomotor appendages, i.e. the very four segments which in the Eurypteridæ are concentrated together to form the endognaths, and we should be justified in putting this interpretation on it, because, according to Kishinouye, its ventral portion forms the coxal gland, and, according to Lankester, the coxal gland sends prolongations into the coxa of the 2nd, 3rd, 4th, 5th locomotor appendages. Similarly in the vertebrate, we find three head-cavities in the region which corresponds, on my theory, to the prosomatic region of Limulus, (1) the anterior cavity discovered by Miss Platt, (2) the premandibular cavity, and (3) the mandibular cavity, which, if they corresponded with the prosomatic cœlomic cavities of Limulus, would represent not three segments but seven segments, as follows:—the anterior cavity would correspond to the first cœlomic cavity, i.e. the cavity of the cheliceral segments in both Limulus and the Eurypteridæ; the premandibular, to the second cœlomic cavity, representing, therefore, the 2nd, 3rd, 4th, 5th prosomatic segments in Limulus and the endognathal segments in the Eurypteridæ; and the mandibular to the 3rd and 4th cœlomic cavities, representing the last locomotor and chilarial segments in Limulus, i.e. the ectognathal and metastomal segments in the Eurypteridæ.
It is worthy of note that, in respect to their cœlomic cavities, as in the position and origin of their nerves in the central nervous system, the first pair of appendages, the cheliceræ, retain a unique position, differing from the rest of the prosomatic appendages.
In the table I have shown how the vertebrate cœlomic cavities may be compared with those of Limulus. The next question to consider is the evidence obtained by morphologists and anatomists as to the number of segments supplied by the trigeminal nerve-group; this question will be considered in the next chapter.
Summary.
In Chapters IV. and V. I have dealt with the opisthotic segments of the vertebrate, including therein the segments supplied by the facial nerve, and shown that they correspond to the mesosomatic segments of the palæostracan; consequently the facial (VII.), glossopharyngeal (IX.), and vagus (X.) nerves originally supplied the branchial and opercular appendages.
In this chapter the consideration of the pro-otic segments is commenced, that is, the segments supplied by the trigeminal (V.) and the eye-muscle nerves (III., IV., VI.). I have considered the VIth nerve with the rest of the eye-muscle nerves for convenience' sake, though in reality it belongs to the same segment as the facial. Of these, that part of the trigeminal which innervates the muscles of mastication corresponds to the splanchnic segments, while the eye-muscle nerves belong to the corresponding somatic segments; but the pro-otic segments of the vertebrate ought to correspond to the prosomatic segments of the invertebrate, just as the opisthotic correspond to the mesosomatic. Therefore the motor part of the trigeminal ought to supply muscles which originally moved the prosomatic appendages, while the eye-muscles ought to have belonged to the somatic part of the same segments.
The first question considered is the number of segments which ought to be found in this region. In Limulus, the Eurypteridæ, and the scorpions there are seven prosomatic segments which carry (1) the cheliceræ, (2, 3, 4, 5) the four first locomotor appendages—the endognaths, (6) the large special appendage—the ectognath—and (7) the appendages, which in Limulus are known as the chilaria, and are small and insignificant, but in Eurypterus and other forms grow forwards, fuse together, and form a single median lip to an accessory oral chamber, which lip is known as the metastoma. Of these appendages the cheliceræ and endognaths tend to dwindle away and become mere tentacles, while the large swimming ectognath and metastoma remain strong and vigorous.
In this, the prosomatic region, the somatic segmentation is not characterized by the presence of the longitudinal muscle segments, for they do not extend into this head-region, but only by the presence of the segmental somatic ventro-dorsal muscles. Among the muscles of the appendages the system of large tergo-coxal muscles is especially apparent.
From these considerations it follows that the number of segments in this region in the vertebrate ought to be seven; that the musculature supplied by the trigeminal nerve ought to represent seven ventral or splanchnic segments, of which only the last two are likely to be conspicuous; and that the musculature supplied by the eye-muscle nerves ought to be dorso-ventral in direction, which it is, and represent seven dorsal or somatic segments.
A further peculiarity of this region, both in Limulus and the scorpions, is found in the excretory organs which are known by the name of coxal glands, because they extend into the basal joint, or coxa, of certain of the prosomatic limbs. The appendages so characterized are always the four endognaths, and it follows that if these four endognaths lose their locomotor power, become reduced in size, and concentrated together to form mere tentacles, then of necessity the coxal glands will be concentrated together, and tend to form a glandular mass in the region of the mouth; in fact, take up a position corresponding to that of the pituitary body in vertebrates.
Taking all these facts into consideration, it is possible to construct a drawing of a sagittal section through the head-region of Eurypterus, which will represent, with considerable probability, the arrangement of parts in that animal. This can be compared with the corresponding section through the head of Ammocœtes.
Now, as pointed out in the last chapter, the early stage of Ammocœtes is remarkably different from the more advanced stage; at that time the septum between the oral and respiratory chambers has not yet broken through, and the olfactory or nasal tube, known at this stage as the tube of the hypophysis, is directed ventrally, not dorsally.
The comparison of the diagram of Eurypterus with that of the early stage of Ammocœtes is remarkably close, and immediately suggests not only that the single nose of the former is derived from the corresponding organ in the palæostracan, but that the pituitary body is derived from the concentrated coxal glands, and the lower lip from the metastoma. The further working out of these homologies will be discussed in the next chapter.
In addition to the evidence of segmentation afforded by the appendages, there are in this region, in Limulus and the scorpion group, three other criteria of segmentation available to us, if from any cause the evidence of appendages fails us. These are—
1. The number of neuromeres are marked out in this region of the brain more or less plainly, especially in the young animal, just as they are also in the embryo of the vertebrate.
2. The segmentation is represented here, just as in the mesosomatic region, by two sets of muscle-segments; the one somatic, consisting of the segmentally arranged dorso-ventral muscles, the continuation of the group already discussed in connection with the mesosomatic segmentation, and the other appendicular characterized by the tergo-coxal muscles. These latter segmental muscles are especially valuable, for in such forms as Mygale, Phrynus, etc., their presence is indicated externally by markings on the prosomatic carapace, and thus corresponding markings found on fossil carapaces or on dorsal head-shields can be interpreted. These two sets of muscle-segments correspond in the vertebrate to the somatic and splanchnic segmentations.
3. In the vertebrate the segmentation in this region is indicated by the cœlomic or head-cavities, which are cavities formed in the mesoderm of the embryo, the walls of which give origin to the striated muscles of the head. In Limulus corresponding cœlomic cavities are found, which are directly comparable with those found in the vertebrate.