Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/670

Rh 636 GEUSTACEA tremely rudimentary organs furnished with hairs which serve for the protection of the eggs after extrusion. In the Macroura they assist in swimming, and are composed of a simple exopodite and enuopodite. The sixth pair are developed into broad plates, forming the lateral lobes to the tail. The Twenty -first or caudal segment is destitute of appendages, and has therefore been considered by Prof. Huxley and others as a &quot;median appendage,&quot; and not as a somite. 1 It varies in form, being sometimes a broad flat plate in the Decapoda, or a minute terminal one in the Amphipoda, or greatly developed as a roof to the branchiae in some Isopoda, or forming a long terminal spine in the Xiphosura. INTERNAL STRUCTURE. NERVOUS SYSTEM. The typical form of the nervous system in the young of all the Articulata is a chain of ganglions placed along the ventral surface of the body, and traversed in front by the gullet. This typical ar rangement, however, undergoes great modifications in the several orders of Crustacea. Taking the Edriophthal- mia, for example (fig. 7), we find the nervous system to consist of two parallel chords traversing the length of the body, each having its own ganglionic enlarge ment in each somite in juxtaposition, but not confluent ; so that there is a distinct pair of ganglions for each seg ment. These are again united by trans verse commissures, and each ganglionic knot gives off nervous, filaments to the limbs of its particular somite. In the lobster (Decapoda-Macrura), the nervous system consists of a longi tudinally disposed series of different- sized ganglions connected together by commissural cords (fig. 8). Primitively there is a pair of gang lions to each somite, but the three first pairs are fused together in the adult so as to form a large cerebral ganglion placed in front of the mouth, and called the supra-oesophageal ganglion. From this a nervous chord passes back on each side of the gullet to the large post- FlG - 76. Nerves of Cy- v u- u J f mottioa. oral ganglion, which is made up of six FIG. a Nerves of iio- pairs of primitive ganglions fused to- marus - gether. Then follow five pairs of thoracic and six abdomi nal ganglions all distinct, but connected one with another by a nervous band formed of the primitive commissural chords whish have coalesced along the middle line. 2 No solid internal skeleton separates this nervous axis from the alimentary system, though reflections of the external integument (apodemata) pass inwards, and more 1 Prof. Bell writes: &quot;Normally there are twenty-one pairs of appendages or limbs ; generally speaking, even in the higher forms, twenty only are perceived, as the terminal joint of the abdomen, which forms the central piece of the fan-like fin, has none which are per ceptible. I have, however, observed them frequently in the common prawn, Pahemon serralus, in the form of extremely minute points attached to the very extremity of the segment, and movable.&quot; Mr A. H. Garrod, F.R.S., is also of opinion that the telson should be regarded as the twenty-first segment, having its appendages modified by cohesion and adhesion. See Humphrey s Journal of Anatomy and Physiology, vol. v. p. 271 (1871). to be composed of two orders of fibres, forming distinct and superposed fasciculi or columns, which the author designates columns of sensation and of motion, analogous to the fasciculi of the anterior and posterior columns of the spinal chord of the higher animals. These fasciculi are but indistinctly discernible in the intergnnglionic chords, but become extremely apparent in the ganglions themselves, for these enlargements belong exclusively to the inferior or sensitive fasciculi, and the superior or motor fasciculi pass over their dorsal surface without penetrating their substance at all. l * ~ or less protect it. From this nervous axis all the nerves are given off, but none arise by two distinct roots like the spinal nerves of man. In the prawn (Paloenwn) and spiny lobster (Palinurus) the thoracic ganglia coalesce to form a long, elliptical, perforated nervous mass. In the hermit-crab (Fayurus) the cephalic ganglion presents a transversely quadrate form, sending off the usual nerves to the eyes, antennae, &c. The lateral cesophageal chords, after supplying the digestive system with the stomato -gastric nerves, unite below to form the ganglion which distributes nerves to the maxillary apparatus and pharynx. This is succeeded by a large oblong ganglion situated at the base of the great nippers, and of the second pair of feet, both of which pairs it supplies. The lateral chords, diverging for the passage of the artery, re-unite to form a third thoracic ganglion, smaller than the second, supplying the third pair of thoracic feet, and sending off three pairs of nerves posteriorly. Of these the lateral pair goes to the fourth diminutive pair of feet ; the median pair supplies the fifth pair of feet ; the two remaining dorsal nerves, which are of minute size, form the continuations of the abdominal chords, and pass along the under or concave side of the soft, membranous, and highly sensitive abdomen to the anus, anterior to which the last small ganglion is situated ; this supplies the nerves to the muscles of the caudal plates, here converted into claspers for enabling the animal to adhere to the columella of the spiral shell which it may have selected to protect the portion of its body undefended by the usual dense and insensible crustaceous covering (Owen). The general progress of the development of the nervous system in the Crustacea has been, as we have seen, attended with increased size and diminished numbers of its central or ganglionic masses. The divisions of each pair of ganglions first coalesce by transverse approximation ; distinct pairs of ganglions approximate longitudinally, conjoining as usual from behind forwards ; confluent groups of ganglions are next found in definite parts of the body, as on the thorax of those species which have special developments and nses for particular legs. In the crab, in which the general form of the body attains most compactness, the ventral nervous trunks are concentrated into one large oval ganglion, from which the nerves radiate to all parts of the trunk, the legs, and the short tail. A corresponding structure of the nervous system is also well displayed in Maia (fig. 9.) An analogous concentration, but not an homologous one, obtains in Limulus. Here the nervous sub stance is chiefly massed round the oesophagus, the fore part of the ring expanding into a pair of ganglions, from which the nerves are sent off to the small median ocelli and the large lateral eyes ; the nerves to the latter are of great length, wind round the anterior apodemata, and bend back to their termination, breaking up into a fasciculus of minute fila ments before penetrating the large compound eye. Two stomato- gastric nerves arise from the upper and fore part of the rino-. From ,1 j / c 1.1. f i. C=ceplialic ganglion ; T=thorao c the under surface of the fore part ganglion; 0,0, optic nerves; a,*, of the ring, a small pair of nerves pass to the first short pair of forci- pated antennules; five large nerves proceed from each side of the ring to the five succeeding jaw-feet. FIG. 9. Nerves squinado, Latr. sntennary nerves ; b, b, r, nto- mato-gastric nerves ; s, medul lary cords uniting C and T; m, maxillary nerves ; g, g, nerves of the flanks; 1,1, nerves of the legs; ab, abdominal nerves. A pair of slender nerves
 * This double gangliouic chain of the lobster was found by Newport