Popular Science Monthly/Volume 21/October 1882/The Past and Present of the Cuttle-Fishes
|THE PAST AND PRESENT OF THE CUTTLE-FISHES.|
FEW groups of the animal kingdom possess a greater interest, either for the zoologist or for the general investigator, than that selected as the subject of the present article. From the earliest ages in which human curiosity concerning external nature began to develop into scientific observation, the cuttle-fishes have formed subject-matter of remark. In the writings of the classic naturalists they receive a due of attention. Their peculiarities of form and habits attracted the notice of Aristotle and Pliny; and even their development, in its more readily observed phases, was studied in the days when biology was but an infantile science. Tracing the lines of cuttle-fish lore onward through the centuries of growing culture, we discern the mediæval spirit of exaggeration and myth seizing upon the group as a likely subject for enlargement and discussion. In the fabulous
history and "folk-lore" of zoölogy, the cuttle-fishes have over and over again played a more than prominent part. In the days of their mythical history they have swallowed whole fleets of ships; they have been credited more than once with the destruction of even an armored navy; and on more than one occasion there can be little doubt that they have played the parts of Sindbad's floating island, and of the "great unknown," the sea-serpent itself. To the modern zoölogist, however, eager in his search after the causes which have wrought out the existing order of animal nature, the cuttle-fishes present themselves as an unusually interesting group.
The definition of the Cephalopoda, or cuttle-fish class, is largely a matter of commonplace observation. Linnæus, naming them "cephalopods," or "head-footed" mollusks, indicated the structural feature which was calculated to appeal most plainly even to non-technical minds. The circlet of arms, feet, or tentacles crowning the head-extremity of a cuttle-fish, thus presents us with a personal character of unmistakable nature. It is necessary, however, to bear in mind that the ordinary and, to a certain extent, natural fashion of representing a cuttle-fish head upward is, in zoölogical eyes, a complete reversion of its surfaces. To understand clearly why to speak of a cuttle-fish head as its lower, and of its tail as its upper extremity, is a correct zoölogical designation, we must enter upon a comparison of the cuttlefish body with the forms of its neighbor mollusks. The contemplation of such a familiar being as a snail or whelk introduces us to a characteristic example of molluscan form and anatomy. The head of the snail or other gasteropod is clearly enough defined; and no less plainly discernible is the enlarged and broadened surface on which the animal walks. This surface is known as the "foot." In one shape or another this "foot" is a characteristic possession of the molluscan tribes. In a section of a mussel or cockle, we perceive the "foot" to exist as a muscular mass developed in the middle line of the body below, and variously used in the mussel class as a spinning organ, a leaping-pole, and a boring apparatus. Here we note the natural development of the foot in the middle line of the animal. Let us suppose this foot to be extended downward, and to be broadened so as to form a surface of progression, and we may conceive readily of the modification whereby a simple foot like that of the mussel becomes developed to form the enlarged disk of the gasteropod. In the latter case we observe that the foot occupies the floor of the body; the bulk of the body, and the head in particular, being borne above.
Cuttle-fish development can be shown to run, so far, in parallel lines to those of the personal evolution of mussel and snail. But divergent paths soon appear in cuttle-fish development; and these variations, while they indicate an ancient departure from the ordinary molluscan type, likewise give to the subjects of our present study their most characteristic features. When a mussel or snail is watched in its earlier stages of development, the embryo is seen, sooner or later, to produce an. appendage highly characteristic of molluscan young at large, and named the velum. By aid of this ciliated fold such an organism as a young cockle, for instance, swims freely through its native waters. This velum undergoes varied changes and alterations in the after-stages of molluscan development; but, when cuttle-fish development is studied in its fullest details, no velum is found among the possessions of the larval body. Such an omission has naturally been made the subject of remark by naturalists. Some authorities—Grenacher, for instance—have insisted upon the recognition of the arms of the cuttle-fish head as the representatives of the missing velum. But, as the latter organ always exists on the dorsal or upper side of the mouth, and as the arms are placed originally behind and under the cuttle-fish mouth, the correspondence of arms and velum has not been accepted by zoölogists. On the other side stands out the opinion of Huxley, who regards the "arms" of the cuttle-fish head as more truly corresponding with the "foot" of the mussel, snail, and other mollusks.
The margins of the foot, in this view of matters, have been prolonged in the young cuttle-fish to form eight, ten, or more arms, and the front and sides of the foot, having overgrown the mouth, are united in front, so that the mouth appears to be placed in the center of the foot, instead of in front and above it, as in other mollusks. So, also, most naturalists maintain, and with every appearance of correctness, that the characteristic "funnel" of the cuttle-fishes—to be hereafter referred to—is an organ formed by two side-processes of the foot, named epipodia. Adopting the view thus sanctioned by competent authority, we may trace in a cuttle-fish the highly modified form of a snail or whelk, and the still more modified form of the mussel tribes. The foot, instead of growing backward and downward as in the snail, and thus forming a broad walking disk, comes to grow over the mouth in front. So that, placing a cuttle-fish in structural comparison with a whelk or mussel, we should have to set it head downward, when the foot (or arms) would be lowest, and the great bulk of the body, with the heart uppermost, would be situated, as in the snail, above the foot.
The group of the cuttle-fishes may be said to divide itself in the most natural fashion into two main divisions. The first of these groups includes all living cuttle-fishes save one—the pearly nautilus. This first division is that of the Dibranchiates, or two-gilled cuttlefishes. The familiar octopus (Fig. 1), the loligos or squids, the sepias, and the argonauts or paper nautili, are among the best known of its representatives. The second group is represented by a single living cuttle-fish, the pearly nautilus (Nautilus Pompilius), just mentioned, and by many fossil and extinct forms.
One of the most remarkable traits of cuttle-fish existence is the curious play of "shot" colors which takes place in their integument. I have seen a loligo, or squid, stranded on the sea-beach make glorious its dying agonies by a play of colors of the most astounding description. The natural purplish tint of the body was now and again deepened to well-nigh a dark blue; the slightest touch served to develop a patch of angry pink; and continually over the whole surface of the body the hues and tints, ranging from dark purple to light red, succeeded each other in rapid array.
The assimilation of an animal's color to the surfaces on which it rests forms a notable circumstance of zoology, which has been denominated "mimicry." That cuttle-fishes possess such a power is well known. The hue of an octopus may so closely resemble that of the rock to which it attaches itself, that the observer can with difficulty say which is rock and which is animal. A flounder's color is in the same way assimilated to the sand on which it rests, although in the fish the alteration of color seen in the cuttle-fishes is not represented.
The manner of production of the changes of hue and play of "shot" colors in the cuttle-fishes is really analogous to that whereby the famed chameleons effect their alterations of hue.
The locomotion of the cuttle-fishes forms a point of interest in connection with their general structure and physiology. Any one who has attentively watched the movements of an octopus in its tank must have been struck by the literally acrobatic ease with which it accommodated itself to the exigencies of its life and surroundings. In their lithe, muscular, and flexible arms, the cuttle-fishes possess an apparatus which is equally serviceable for the capture of prey, and for walking mouth downward—that is, in their structurally natural position. They possess, likewise, the power of swimming upper side forward—or popularly stated "backward"—by means of the jets of water which, by forcible contractions of the muscular mantle-sac, are projected from the tube or "funnel" situated on the hinder face of the body. These jets d'eau consist of the effete water which has been used in breathing, so that the act of expiration and the effete water of respiration together become utilized, in the economical wisdom of nature, as a means of propulsion. The mysterious backward flight of an octopus through its tank (Fig. 1), when, detaching itself from its hold on the rock, it swims gracefully and swiftly through the water, is effected in the manner just described. This form of hydraulic apparatus, imitated in experiments in marine engineering, serves but to strengthen the wise man's adage concerning the utter lack of novelty in terrestrial and mundane things.
It is equally interesting to note that some of the squids or loligos—named popularly "flying squids"—appear to be able to rise from the surface of the sea and to spring into the air after the fashion of the flying-fishes. Instances are mentioned of the flying squids having occasionally landed themselves on the decks of ships in their atmospheric leaps.
The "arms" or "feet" demand, however, a somewhat detailed mention, on account of their armature. In all cuttle-fishes, save the exceptional pearly nautilus, the arms are either eight or ten in number, and are provided with acetabula, or "suckers." Those cuttles in which ten arms are present and of these the squids and sepias form good examples—have two of these appendages produced beyond the remaining eight in length. The "suckers" (Fig. 2, a), which constitute a most noteworthy armament of the arms, are borne on short stalks in the ten-armed cuttle-fishes, but are unstalked in the eight-armed species. Each sucker (Fig. 2) exhibits all the structures incidental to an apparatus adapted to secure effective and instantaneous Fig. 2.—Suckers of the Cuttle-fish. adhesion to any surface. It consists of a horny or cartilaginous cup (a), within which are muscular fibers converging toward its center, where they form a well-defined plug or piston (b). By the withdrawal of this plug a partial vacuum is produced, and the suckers adhere by atmospheric pressure to the surface on which they are placed. The sucker is released by the projection of the plug and by the consequent destruction of the vacuum. The number of the suckers varies, but is always considerable; and when we reflect that the array of suckers can be instantaneously applied, and that their hold is automatically perfect, the grasp of the cephalopods is seen to be of the most efficient kind. In some cuttle-fishes, and most notably in the so-called "hooked squids" (Onychoteuthis), the pistons of the suckers are developed to form powerful hooks, by means of which the prey may be secured with additional facility; and in the common squids the margin of the sucker is provided with a series of minute horny hooks. The "arms" themselves, it need hardly be remarked, are extremely mobile; they are highly muscular, and can be adapted with ease to the varied functions of prehension and movement they are destined to subserve. As regards their arrangement, they are arranged in four pairs—a dorsal and a ventral pair, and two lateral pairs; the two elongated tentacles, when developed, being situated between the third and fourth pairs of arms on the ventral or lower surface.
The alimentary tract or digestive system of the cuttle-fish race is in every respect of well-developed and complete character. Lower down in the molluscan series the commissariat department is subserved by a very perfect digestive apparatus, including representatives of most of the organs familiar enough to us in higher or vertebrate existence. In the cephalopods we should naturally expect the standard of lower-molluscan organization to be further elaborated; and this anatomical expectation is justified by the actual details of cuttle-fish structure. The mouth opens on the upper surface of the head—a disposition of matters already accounted for when considering the relations of the cuttle-fish body to that of other mollusks. The mouth-opening is usually bounded by a raised lip, and leads into a cavity containing an elaborate apparatus, analogous to the jaws of higher animals, and by means of which the food of these animals is triturated and divided. An inspection of the masticating apparatus of a cuttle-fish readily solves the question, "How are the hard shells of their crustacean food broken down?" There exists within the mouth, firstly, a hard, horny beak, resembling closely in shape the beak of a parrot, and consisting of two chief divisions, whereof one—the front—is the smaller, and is overlapped by the hinder beak. Set in action by appropriate muscles, these beaks divide the hard parts of. the food with the greatest ease. But a second apparatus of more typical nature likewise exists in these animals. This is the odontophore, a structure popularly named the "tongue," and which is common to the whelk and snail class, to the sea-butterflies, and to the cuttle-fishes. It consists essentially of an elongated ribbon-like structure, bearing hooked teeth, generally disposed in transverse rows. This apparatus, set in action by special muscles, and worked after the fashion of a chain-saw, is used to rasp down the food; while new growths of its substance from behind serve to repair the loss caused by the friction to which it is subjected.
The gills, as already noted., number two in all cuttle-fishes except the pearly nautilus, and may demand a special notice. Each gill is a conical organ, consisting essentially of a dense net-work of blood-vessels, in which impure blood brought by the great veins is exposed to the action of the oxygen contained in the water which is being continually admitted to the gill-chambers. Each gill is contained within a kind of chamber to which water is admitted by the front edge of the mantle-sac. This opening being closed by a valve against the exit of the water, the forcible contraction of the body-walls ejects the water, as previously described, from the "funnel." The gills are themselves contractile, but they do not possess the armament of minute vibratile processes or cilia, so typical of the gills of other mollusca. The need for these cilia as organs providing for the circulation of water over the gill-surfaces is of course removed, in view of the very perfect means existent in the cuttle-fishes for the renewal of the water used in breathing. As a living octopus or other cuttle-fish is watched, the movements of inspiration and expiration are plainly indicated by the expansion and contraction of the body-walls, and they imitate in a singularly exact fashion the analogous movements of the highest animals. Observers have likewise described in certain members of the cuttle-fish class a series of minute pores, by which water enters the great veins and mixes with the blood. It is also certain that water enters the general body cavity and bathes the organs of the animal, thus converting that cavity into a physiologically active space, possessing an influence on the circulation in that its contained water presents a medium for the conveyance of oxygen into, and for the reception of waste materials from, the blood.
Connected on the one hand with the digestive system, and on the other with the more purely glandular structures of the body, is the organ known familiarly as the "ink-bag" of these animals. The cuttlefishes are well known to utilize the secretion of this sac as a means of defense, and for enabling them to escape from their enemies. Discharging the inky fluid through the "funnel," into which the duct of the ink-sac opens, it rapidly diffuses itself through the water, and enables the animal to escape under a literal cloak of darkness. The exact nature and relationship of this ink-sac to the other organs of the cuttle-fish have long been disputed. According to one authority, the ink-bag represented the gall-bladder, because in the octopus it is imbedded in the liver. From another point of view, it was declared to represent an intestinal gland; while a third opinion maintained its entirely special nature. The ink-sac is now known to be developed as an offshoot from the digestive tube; and, taking development as the one infallible criterion and test of the nature of living structures, we may conclude that it represents at once a highly specialized part of the digestive tract, and an organ which, unrepresented entirely in the oldest cuttle-fishes, has been developed in obedience to the demands and exigencies of the later growths of the race. It is this ink-sac which is frequently found fossilized in certain extinct cuttle-fish shells. Its secretion forms the original sepia color, a term derived from the name of a cuttle-fish genus. The fossilized sepia has been used with good effect when ground down. The late Dean Buckland gave some of this fossil ink to Sir Francis Chantrey, who made with it a drawing of the specimen from which it had been taken; and Cuvier is said to have used this fossilized ink in the preparation of the plates wherewith he illustrated his "Mollusca." At the present time, recent cuttle-fish ink is said to be utilized in the manufacture of ordinary artists* "sepia."
The due regulation of cuttle-fish existence is determined by the action of its nervous apparatus. The ordinary type of molluscan nervous system undergoes in the cuttle-fishes a decided change of form. In a snail or whelk, for example, the nervous system exhibits an arrangement of three chief nerve-masses or "ganglia," connected by nervous cords. Of these three nerve-centers, one is situated in the head, a second in the "foot" or organ of movement, and a third in the neighborhood of heart and gills, or amid the viscera generally. Increased concentration of this type of nerve-arrangement awaits us in cuttle-fish organization. Just as the spider possesses a more concentrated and localized nerve-axis than the insect, or as the gangliated chain of the latter becomes the fused nerve-mass of the spider, so in the cuttle-fish, the molluscan nerve-system, scattered and diffused in the snail, whelk, or mussel, becomes localized in adaptation to the increased nerve-control and to the wider instincts of cuttle-fish existence. This process of nerve-localization and concentration is accompanied by certain important modifications affecting other regions and structures of cuttle-fish economy. Thus the nerve-centers are found to be protected and inclosed within a gristly or cartilaginous case, that foreshadows the functions of the vertebrate skull, though in no sense connected with that structure.
Not the least interesting feature of this localized mass of nervous matter is the fact that it exhibits the same arrangement of gray and white nerve-matter that is seen in the highest brains. An outer gray and an inner white layer are discernible in the nerve-ganglia of cephalopods, as in the cerebrum of man; and, as in the highest animals, the cuttle-fish gray matter is found to consist of nerve-cells, while the white matter is chiefly composed of nerve-fibers. Thus the laws of developmental progress affect the microscopic and intimate structure of the living form as well as the more obvious details of structure. From the main nerve-mass of the cuttle-fishes nerves arise to supply the body at large. Nerves of special sense supply eyes, ears, and olfactory organs; while the viscera and the "mantle" or general body-covering are also well provided with the means of innervation.
Cuttle-fish existence possesses, in all probability, the five "gateways of knowledge," through which the impressions of the outer world are received, and by which these impressions are modified and transmitted to the brain-masses as sensations of sight, hearing, smell, touch, and taste. There is little need to draw upon hypothesis in the assumption that the arms or tentacles are efficient organs of touch in Cephalopoda, or that the structures of the mouth may subserve taste, in so far as the latter sense may be required to satisfy the demands of cuttle-fish existence. An organ of smell is definitely situated behind or above the eyes. There two small projections, or, as frequently, two minute pits or depressions, occur. These pits are ciliated, and between the cilia "olfactory cells" are situated. These cells, in turn, represent the similar structures which occur in higher animals, and which, in man himself, form the characteristic terminations to his olfactory nerves. That the cuttle-fishes can literally scent their prey from afar off is an idea confirmed by the facts of their every-day life.
The "ears" of the cuttle-fishes present us with two sacs named "auditory sacs"—which may, as in the nautilus, either be attached to the chief nerve-mass itself, or, as in the two-gilled cuttles, be lodged in special cavities in the gristly "skull." A cuttle-fish "ear" is essentially a sac or bag, called an "otocyst," containing either one or many "otoliths" or "ear-stones," suspended in a watery fluid. This, indeed, is the primitive type of "ear" we may find even in the Medusidæ or "jelly-fishes" themselves. The ear-sacs of many cuttlefishes open on the external surface of the body by two fine canals, named "Kölliker's ducts," after their distinguished discoverer. Occasionally these ducts end blindly, and do not open on the body surface. These facts lend additional support to the opinion that in the ear of the cuttle-fish we find primitive structures proper to the ears of vertebrates, the minute canals of Kölliker corresponding with the recessus vestibuli of the vertebrate organ of hearing. Once again, therefore, we find the progressive development of cephalopods and vertebrates running in parallel, but nevertheless in distinct and independent, lines; and this likeness is further strengthened when we discover that not merely the ear, but the eye likewise, of these two groups of animals is formed or developed in an essentially similar fashion. The ear of the cuttle-fish presents us with a permanent example of an early and transitory stage in the development of the vertebrate ear, and a common plan of ear-production is thus seen to traverse a wide extent of the animal world.
The present history of the cuttle-fishes may be concluded by the briefest possible reference to their distribution and classification. Over two thousand species of cephalopods are known. But geology claims the vast majority, only two hundred and eighteen species being included in the ranks of living animals. The cuttle-fishes are very widely distributed in existing seas. They occur in the far north; they are plentifully represented in the colder seas by the squids which form the bait of the Newfoundland cod-fishers; but in tropical regions they attain their greatest size and numerical strength. Their classification is both simple and natural. Their division into Dibranchiates ("two-gilled") and Tetrabranchiates ("four-gilled") is a method of arrangement which accurately reflects variations in their existing structure, as it correctly indicates the main lines of their geological and past history. Of four-gilled cuttle-fishes there is but one living example—the pearly nautilus (Fig. 3). Its special and distinctive peculiarities may be rapidly summed up in the statement that it has
four gills, numerous arms (c), no suckers, no ink-sac, an incompletely tubular funnel (f), stalked eyes, and an external, many-chambered shell, in the last formed and largest compartment (e) of which the body is lodged.
The absence of| an ink-sac in the nautilus is a fact correlated with its bottom-living habits and with the absence of any need or requirement for the sudden concealment from enemies which the more active two-gilled forms demand. The many-chambered shell of the pearly nautilus exhibits a flat, symmetrical, spiral shape. Its many-chambered state is explained by the fact that as the animal grows it successively leaves the already formed chambers, and secretes, a new chamber to accommodate the increasing size of body. Each new chamber is partitioned off from that last occupied by a shelly wall called a septum (g). Through the middle of the series of septa runs a tube named the siphuncle, (s, s), whose function has been credited with being that of maintaining a low vitality in the disused chambers of the shell.
All other living cuttle-fishes possess, on the contrary, two gills, never more than ten arms provided with suckers, an ink-sac, unstalked eyes, a completely tubular funnel, and an internal shell. If, however, the nautilus represents in its solitary self the four-gilled cuttle-fishes of to-day, it likewise, like "the last of the Mohicans," appears as the descendant of a long line of famous ancestors. In its distribution, the nautilus is limited to the southern seas. It is still the rarest of animals in our museums, although its shells are common enough.
This, according to Mr. Moseley, is no doubt due to the fact that the animal is mostly an inhabitant of deep water. The shells of Spirula (Fig. 6) similarly occur in countless numbers on tropical beaches, yet the animal has only been procured two or three times.
It is thus the pearly nautilus floats under certain, circumstances on the surface of the water. The argonaut (Fig. 4), credited in poetry and fiction with this power, never floats on the surface, as was of old believed. It is simply a mundane cuttle-fish, whose two expanded arms are never used as sails, after the popularly supposed fashion, but are employed solely to secrete and attach to the body the false shell (Fig. 4, a) with which it is provided.
Among the two hundred odd living two-gilled cuttle-fishes, considerable diversity of external form may be seen; but the general type
already described is at the same time closely adhered to; and save in the case of the paper nautilus or argonaut, in which the characteristic shape of body is concealed by the shell, the cuttle-fish characters are readily apparent. The shell of the paper nautilus (Fig. 4, a) is termed "false" or "pedal," because it is not formed by the mantle, as all true shells are, but by the two expanded arms, as already mentioned. In its homology it therefore coincides with foot-secretions (such as the "beard" of the mussel), and not with the shells of its neighbors. The female argonaut alone possesses a "shell," the male (Fig. 4, c) being a diminutive creature, measuring only an inch or so in length. It is in the ranks of the two-gilled cuttle-fishes that we discover those phases of cuttle-fish life which most characteristically appeal to the popular mind. Thus, many species of two-gilled cuttles are eaten and considered dainties by foreign nations; it is from this group that the sepia color already mentioned is obtained; their internal shells gave us the "pounce" of long ago, and formed an article in the materia medica of by-gone days; and, lastly, it is in this group that the mythical and the real meet in the consideration of the giant cuttle-fishes which the myth and fiction of the past postulated, and which modern zoölogy numbers among: its realities.
The past history of the cuttle-fishes unites in itself a knowledge at once of their present position in the animal world and of their progress toward that position. The history of their past begins with the recognition of the pearly nautilus (Fig. 3) as a being which, as a four-gilled cuttle-fish possessing an external many-chambered shell, stands alone in the world of life. It is the tribes of two-gilled cuttle-fishesFig. 6.—Spirula. which people our ocean to-day, and which exhibit ail the gradations of form and size, from the minute Spirula (Fig. 6) to the great Architeuthis of the American coasts. The history of the cuttle-fishes in time begins in the far-back epoch represented by the Lower Silurian rocks of the geologist. There are entombed the first fossil cuttle-fishes, represented by their chambered shells. The genus Orthoceras, represented by shells of straight form, is thus among the oldest members of the cuttle-fish race. The Nautilus genus itself begins in the Upper Silurian rocks; we may trace the well-known shells upward to the Carboniferous strata, where they are best developed; and we follow the genus onward in time, as it decreases in numbers, until we arrive at the existing order of things, in which the solitary nautilus remains, as we have seen, to represent in itself the fullness of cephalopod life in the oceans of the past. The older or Palæozoic rocks reveal a literal wealth of these chambered shells, and therefore of the existence of the four-gilled cuttle-fishes as the founders of the race. When we ascend to the Mesozoic rocks (ranging from the Trias to the Chalk), we meet with new types of the chambered shells well-nigh unknown in the Palæozoic period. In the Mesozoic rocks appears the fullness of Ammonite life. Here we find shells named after the horns of the Egyptian god, Jupiter Ammon; these, instead of being tolerably plain, like the Nautilidœ, exhibit beautifully sculptured outlines, and folded septa, or partitions, between the chambers of the shell. The shells allied to Nautilus and occurring in the Palæozoic formations differ from Nautilus chiefly in their varying degrees of curvature or straightness. Lituites is a curved form allied to Nautilus / while Orthoceras and Gomphoceras are groups representing the straightened forms. But in the Silurian period more complex forms appear, with elaborate and folded septa. These are the early Ammonites, such as Goniatites and Bactrites. In the Secondary rocks we find the still more complex true Ammonites themselves. Here the lobes and saddles of the shells, as the edges of the septa are named, are of the most elaborate patterns, while the shapes of shell are of the most varied character (Baculites, Turrilites, Ammonites, etc., Fig. 5).
There is thus an advance and progression exhibited in the development of the four-gilled races which accords perfectly with the theory of evolution and descent. The seas of the Trias, Oolite, and Chalk periods must have literally swarmed with these striking forms of cephalopod life; but as the close of the Chalk period dawned, and as the Secondary age came to an end, the fullness of the Ammonite generations disappeared for ever. In the succeeding Tertiary period not a single Ammonite of any kind occurs; the genus Nautilus remaining in the Tertiary period—as it survived into the Mesozoic or middle period—as the sole representative of a once plentiful four-gilled population.
If the history of the four-gilled cuttle-fishes is thus plainly told as having its beginnings in the Palæozoic period, its maximum development in the Mesozoic period, and its lingering presence in the Tertiary period, the two-gilled cuttle-fishes may be said to possess an equally interesting history. Compared with their four-gilled neighbors, the two-gilled forms are late comers upon creation's scene. Not a single fossil two-gilled form occurs in all the Palæozoic period, extending from the Laurentian to the Permian rocks. If they existed in Palæozoic seas, they have at least left no trace of their presence. Their softness of body may perchance have contributed to their elimination from the oldest fossil records; but, laying aside mere conjecture, we find the first fact of the past history of the two-gilled forms in the presence of the fossil shells of the extinct Belemnites in the Triassic rocks. The Belemnites themselves disappear at the close of the Mesozoic period; but fossilized shells of species allied to our living Sepias occur in the Oolite; and the internal shells of squids are found in the Lias or lower Oolites. In the Tertiary rocks, Argonaut (Fig. 4) shells occur in the Pliocene deposits; the Eocene rocks also give us sepia remains; and various other two-gilled fossils (Beloptera, etc.) are found in Eocene and Miocene formations.
Briefly summarized, then, we find that the chief details in the past history of the cuttle-fishes are told when we are reminded that the four-gilled forms are by far the more ancient of the two groups; that they first appear in the Silurian rocks, while the two-gilled forms appear first in the Secondary rocks; and, lastly, that the record of the one group is the converse of the other. For, the four-gilled species attained their maximum in the Primary and Secondary rocks, and have practically died out, leaving the pearly nautilus as their sole representative in existing seas. The two-gilled race, starting in the Secondary rocks, and leaving the extinct belemnites as a legacy to the past, have, on the other hand, flourished and progressed, and attain their maximum, both in size and numbers, in the existing seas and oceans of our globe.
What ideas concerning the origin and evolution of these animals may be legitimately deduced from the foregoing facts of their structure and distribution in time? In the answer to such a question, asked concerning any group of living beings, lies the culminating point of all biological science. That the cuttle-fishes fall nominally into their place in the scale of being indicated by evolution, and that in their individual development, in the growth of their special organs, such as eye and ear, as well as in the general relations they bear to each other as living forms, they illustrate the results of progressive development, can not for a moment be doubted. The further fact that the existing four-gilled nautilus, despite its lengthy ancestry, as regards its brain, its eye, its tentacles, and other features of its history, is a less specialized and lower form than the two-gilled cuttle-fishes, clearly points to the evolution of the two-gilled from the four-gilled stock. The more active and structurally higher races of to-day, in other words, have sprung from the less specialized and lower cuttle-fishes of the geological yesterday. No question, then, of the reality of progressive development, as a factor in evolving new species and groups of cuttle-fishes from the confines of already formed species, can be entertained.
Turning more specifically to the shell in general, we may discover in the modifications of this single structure a clew to the entire evolution of the cuttle-fish race. The "shells" of the two-gilled cuttle-fishes exist for the most part as horny "pens" or as limy plates, secreted by the "shell-gland" of the mantle which forms the true shell of all mollusks. Starting with the shells which are certainly oldest in point of time, and therefore of development, we find, in the Nautili and their neighbors, structures which represent fullness of shell-growth. It appears a long hypothetical journey from the well-developed shell of the nautilus type to the limy plate or horny "pen" shell of the squid. But the halting-places on the way diminish the apparent length of the journey, as they lessen the seeming irregularity of the path. The simple rudimentary shells of our two-gilled cuttle-fishes are to be regarded as the degenerate remains of structures fully developed in their ancestors. To this idea, their succession in time bears faithful witness; and to its correctness the connecting links, accessible to us, plainly testify.
Thus the history of the cuttle-fish shell forms an important chapter in the biography of the race. The rudimental shells of the two-gilled cuttle-fishes, like the teeth which never cut the gum in unborn whales, have a reference not to their present life, but to a former state of things. Contemplating the "pen" or "cuttle-bone" of a modern squid or sepia, our thoughts become molded in mental continuity with the past. There rise to view before our mind's eye the ancient Nautili and their sculptured kith and kin the ammonites, crowding the seabeds of the far-back Mesozoic, and still more remote Palæozoic ages. Then, through the operation of the inevitable laws of organic progress and advance—making the ancient world then, as they constitute our world to-day, the theatre of continual change—we see the two-gilled stock arise in secondary times from the four-gilled race. First there is seen the modification of shell. Concurrently with the decrease of shell comes increase of head-development and elaboration of nerve centers, tending to make the new two-gilled form what we know it to be to-day—the wary, watchful organism, living in the waters above, and occupying a sphere of vital activity immeasurably superior to the dull existence passed by its four-gilled ancestors on the ocean-bed. The shell degenerates more and more as the cuttle-fish race rises on its own branch of the animal tree. Development in numbers succeeds individual advance. The cephalopod tribes of to-day dawn fuller and fuller as the Tertiary period progresses. Thus the fullness of cuttle-fish life to-day, exhibited in all its strange weirdness, is interwoven, like the lines of human history itself, with the warp and woof of the past. And not the least important clew to the history of that past is found in the apparently insignificant "shell" we have discussed; since in its mere degeneracy it leads us backward in an instructive glance to those early times when the chief branches on life's tree had not reached their full fruition, and to the days when the world itself was young.
- Abridged from "Belgravia."