236

NERVOUS SYSTEM OF CRUSTACEA.

locomotion; but in those which are destined to walk rather than to swim, this part is generally very small and bent under the thorax. In the Crabs, for instance, this portion of the body is reduced to almost nothing, and forms a sort of moveable appendage, which is seen on the inferior surface of the body beneath the legs (Fig. 458).

774. The Nervous System of Crustacea consists of a double series of ganglia, situated on the ventral surface of the body, near the central line. In general their number corresponds to that of the distinct segments composing the body; and the first pair is always

placed on the head, in front of the œsophagus, where it forms a sort of brain. The arrangement of the thoracic and abdominal ganglia, however, varies considerably; sometimes they are placed at equal intervals, and form, with their cords of communication, a chain extending from one end of the body to the other; sometimes they approach one another more or less closely; and sometimes they are united, forming a

single mass, situated about the middle of the thorax (Fig. 461, t). It may be remarked that this concentration of the nervous system becomes more and more complete, as the animal acquires a higher organisation. The Crustacea have in general but very limited faculties; and none among them present much to interest, as to their habits. The eyes are formed nearly in the same manner as in Insects; sometimes they are simple, but in general they are compound; and in all the most perfect Crustacea, these

ORGANS OF SENSE OF CRUSTACEA.

237

organs are situated on moveable footstalks (Fig. 462); an

arrangement which we do not see in any other division of

ai

a

m

FIG. 463-Anterior part of the inferior surface of the body of a Crab (Maia):

Articulated animals.-In many Crustacea, there is an apparatus for hearing, situated at the base of the external antennæ, and composed of a small membrane, under which we find a sort of vestibule filled with liquid, and inclosing the termination of a particular nerve. This consti

ai, internal antennæ; a, external tutes the simplest form of an

antennæ; y eyes; o, organ of hear

ing; m, feet jaws; b, mouth; p, base Auditory apparatus. (ANIM. of the anterior limbs; r, entrance to PHYSIOL. § 512). We know

the respiratory cavity; s, sternum.

nothing positive concerning the

senses of Smell and Taste in these animals.

775. Most Crustacea subsist on animal substances; but they present great differences in their regimen, some being only nourished upon liquid matters, whilst others feed upon solid aliment; and we observe corresponding differences in the formation of their mouths. In masticating Crustacea there is in front of this opening a short transverse lip, followed by a pair of mandibles, by an inferior lip, by one or two pairs of jaws properly so called, and generally by one or three pairs of auxiliary jaws, or feet-jaws, which serve principally for the prehension of food

238

MOUTH AND DIGESTIVE APPARATUS OF CRUSTACEA.

(Fig. 472). In suctorial Crustacea, on the contrary, the mouth is prolonged into a sort of beak, or trunk, as we have already seen in Insects of the same habits. In the interior of this tube, we find thin and pointed appendages, which perform the office of little lancets; and on each side we commonly see organs, which are analogous to the auxiliary jaws of masticating Crustacea, but which are so formed as to serve to fix the animal on its prey.

776. The digestive canal extends from the head to the posterior extremity of the abdomen; and is composed of a very short œsophagus, a large stomach (e, Fig. 466) generally armed interiorly with powerful teeth, of a small intestine, and of a rectum. In some Crustacea, the bile is secreted by biliary

FIG. 464.-ANATOMY OF A CRAB; the greater part of the carapace having been removed; p, portion of its lining membrane; c, heart; as, ophthalmic artery; aa, abdo. minal artery; b, branchia in their natural position; b, branchiæ turned back to show their vessels; fl, lower portion of the shell; f, appendage of the foot-jaw; e, stomach; m, muscles of the stomach; fo, liver.

vessels, like those of Insects; but in general there is a very voluminous liver, divided into several lobes, and composed of a

CIRCULATION AND RESPIRATION OF CRUSTACEA.

239

multitude of small tubes terminating in follicles, and grouped round a ramified excretory canal, whose extremity empties itself on each side into the intestine near its commencement (fo, Fig. 464.)

777. We know as yet nothing of the manner in which the chyle passes from the intestine into the circulating apparatus. The blood is colourless, or slightly tinged with blue or lilac; and coagulates easily. This liquid is put in motion by a heart, situated on the median line of the back, and composed of but

FIG. 465. CIRCULATING APPARATUS OF LOBSTER; a, heart; b and c, arteries to the eyes and antennæ; d, hepatic artery; e and f, arteries to thorax and abdomen; gg, venous sinus; h, gills; i, branchial veins.

and

a single cavity of variable form (Fig. 465, a). Its contraction. propels the blood into the arteries, which distribute it to all parts of the body. The veins are very incomplete, and are formed chiefly by passages left between the different organs, lined by a thin layer of areolar tissue; they end in large cavities near the base of the legs (Fig 466, s,) from which the blood is conducted to the respiratory organs, whence it returns to the heart by very distinct canals termed branchio-cardiac vessels.

778. Crustacea are almost all essentially aquatic; their respiration is nearly always effected by gills; and when these organs are absent, their place is supplied by the skin of certain parts of the body, generally of the legs. In other respects, the arrangement of the respiratory apparatus varies considerably. Thus in the Crabs, Cray-fish, and all other Crustacea of analogous organisation, the gills consist of a considerable

d

240

RESPIRATORY APPARATUS OF CRUSTACEA.

number of pyramids,-each composed either of a number of minute cylinders placed like the hairs of a brush, or of little

lamellæ piled one on the other like the leaves of a book.These organs are fixed by their extremities to the inferior border of the arch of the flanks (Fig. 466); and are inclosed in

two large cavities, situated at the side of the thorax and shut in between the carapace

and the arch just men

tioned, an arrangement which is not found in the other divisions of this class. The respiratory cavity communicates with the exterior

FIG. 467.-RESPIRATORY APPARATUS OF PRAWN; a, rostrum or beak; b, carapace; c, base of antennæ; d, base of abdomen; e, base of legs; f, gills; g, dotted line, marking the turned-in border of the portion of the carapace that covers in the gills, which has been removed in this preparation; h, canal for the exit of the water from the respiratory chamber; i, its valve; j, its extremity.

by two openings; that which serves for the entrance of the water is situated between the base of the legs and the side of

C