Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/671

CRUSTACEA 637 pass to the spiny-edged lamelliform appendage. The posterior part of the nervous ring is prolonged backwards in the form of a chord having four ganglionic enlargements on its ventral surface, and terminates opposite the penulti mate post-abdominal plate in a fifth slight ganglionic enlarge ment which bifurcates ; each division sends off a few nerves as it proceeds to the caudal appendage, on entering which it is resolved into a plexus or kind of cauda equina. Besides the principal nerves above mentioned many smaller nerves are given off to other parts of the body. The sides of the great oesophageal ring are united by two transverse com- missural bands ; but the most remarkable feature of the nervous axis of this crustacean is its envelopment by an arterial trank. A pair of aorta from the fore part of the heart arch over each side of the stomach, and seem to terminate by intimately blending with the sides of the cesophageal nervous ring. They, in fact, expand upon, and seem to form its neurilemma ; a fine injection thrown into them coats the whole central mass of the nervous system with its red colour (Owen, Lectures Invertebrata, 1855). Tracing the nervous system in the Crustacea from its simplest typs, a vermiform cord with a series of independ ent nerve centres, we see these becoming successively con joined in a greater and greater degree, as if in obedience to Some law of attraction, until in the crab the maximum Centralization of the class is attained. But in whatever form it exists in this section of the Arthropoda we may bear in mind the conclusion that "the nervous system of the Crustacea consists uniformly of medul lary nuclei (ganglions) the normal number of which is the same as that of the members or rings of the body, and that all the modifi cations encountered, whether at different periods of the incubation, or in different species of the series, depend especially on the approxi mation, more or less complete, of these nuclei (an approximation Which takes place from the sides towards the median line, as well as in the longitudinal direction), and to an arrest of development occurring in a variable number of the nuclei." (Milne-Edwards.) DIVISIONS OP THE NERVES IN THE CRUSTACEA. Three principal divisions have been recognized in the nervous system of the Crustacea : (1.) All those nervous filaments which take their rise in, and are exclusively connected with, the supracesophageal nerve-centre, form ing the true sensori-voHtional system. (2.) Other ganglions super- added to the abdominal columns with their nervous filaments, serving for the automatic reception and reflection of stimuli, forming the motor system. (3.) The stomato-gastric nerves, connected partly with the brain and partly with the cesophageal columns (analogous to the great sympathetic or organic nerves of the Vertebrata), form ing a third group, the ganglionic system. SEAT OF THE SENSES IN THE CRUSTACEA. Although, as regards the relative size of the several ganglions in the nervous chain of the Crustacea generally, there is little difference between the anterior and posterior masses, and often a disparity between the supra- as compared with the infra-oesophageal ganglions/ yet, nevertheless, it is gener ally admitted that in these animals there is an evident tendency observable towards a centralization of the nervous functions in the anterior portion of the ganglionic chain, viz., the supra-ossophageal ganglion. But still there is a wide interval between this first indication and the concen tration of the faculties of perception and of will in a single organ the brain of which every other portion of the ner vous system then becomes a mere dependency. ORGANS OF FEELING. As regards the development of the individual senses, one may reasonably conjecture that the sense of touch can be but feebly exercised by the com mon integument of the Crustacea, if indeed it can be said to exist at all, except in those parts of the body which remain soft and undefended by a calcareous crust, such as 1 In Maia squinado, for example, although the supra-oesophageal or tephalic ganglion is large, yet, in consequence of the union of the cephalo-thoracic somites, the thoracic ganglion is fully three times its size. See su_pra, fig. y. the under side of the abdomen or the soft body of the hermit-crabs. The hairs with which many of the Crustacea are indued may to some extent compensate for this low endowment of the tactile sense. There can be no doubt, however, that the sense of touch is mainly concentrated in the two pairs of long, many- jointed, and highly flexible antennae with which numbers of this class are provided. These special organs of touch are directly connected with the cephalic ganglion, and are well adapted, both by actual exploration and as media for conveying vibratory sensations, to furnish to the brain most correct and rapid ideas of surrounding objects. The smaller but similarly-formed flabelliform appendages attached to the maxillag and maxillipeds doubtless perform a similar office in the testing of all objects brought near to the mouth ; these latter, however, are not directly connected with the supra-oesophageal, but with the mandibular ganglion. ORGANS OF SIGHT. The eyes of Crustacea present a greater variety in their gradation than is to be found in any other class of the Arthropoda. Commencing with only a median fixed (bifid ?) eye-spot in the larval and simpler forms, we see these organs advance progressively, through all the stages of sessile-eyed development in the Merostomata and Edriophthalmia, to the highest condition in the Podoph- thalmia, that of two distinct compound eyes, endowed with all the essential optical apparatus, and placed on mov able peduncles. It has been doubted by some natu ralists whether the eyes are organs of f~ so much importance in the economy of the Crustacea as are the antennae or organs of touch ; but experiments per formed on the eye of the living lobster, when out of water, or even in a shal low aquarium, can hardly be deemed as either a conclusive or a satisfactory test of the sensitiveness of the cornea in an eye accustomed to convey im pressions of surrounding objects to the optic nerve when at a depth of several fathoms beneath the water. If the open-air experiments as to the sensitiveness of the lobster s eye had Fig. 10. FIQ. 10. Structure of Eye of Lobster (Homarus vulyaris}. After Newton. la. Longitudinal and horizontal section of a right eye seen by reflected light (X 4). a, cornea; 6, first band of plument, beneath which are the crystalline cones; c, a broad band of radiating fibres free from pigment; rf, second bliick band composed of the pigmented spindle-shaped bodies, the lower ends of these bi>dies are covered with an opaque white pigment which forms e, the first white band; h, bundles of radiating fibres; t, enlarged end of the optic nerve, x. The muscles and connective tissue which naturally fill the cavity have been omitted in this figure. 16. A group of elements showing the relation of the pigment to the cones. The cornea is not present, a, sul.stance intermediate between cornea and cone ; fr, crystalline cone; c, nerve rt.J ; p, pigment. Ic. A partly diagramma ie view of one of the elements of the eye from the cornea to the optic ganglion, o, coinea; 6, substance between cornea and cone; 6 . lower end of ciystalline cone; c. nerve rod, around which is seen the Investing membrane with its nuclei ; d, spindle-shaped or transversely striated body ; /, perforated membrane at surface of optic ganglion; p, pigment. Id. A portion of cornea as seen by reflected light, showing the cross and central spot. le. Perpendicular section from the middle of the cornea, showing the smooth outer and slightly convex inner surfaces. been made instead on a living shore or land crab, of the genus Grapsus, Gelasimus, Gecarcinus, or Ocypoda, it would speedily have been found that in point of rapidity of per ception and movement, guided by sight, these shore and land crabs are quite equal to the most sharp-sighted insect, or the most agile of lizards. As already stated, the eyes are the most constant and

persistent organs possessed by the Crustacea as a class ;