762 



CRUSTACEA. 



place, the only kind upon which it seems neces- 

 sary to say anything here, are effected in two 

 modes, either by the alternate flexion and ex- 

 tension of the trunk, or by the play of the limbs. 



In those Crustacea which are formed essen- 

 tially for swimming, the posterior part of the 

 body is the principal agent in enabling the 

 animal to change its place ; but here the mo- 

 tions, instead of being lateral, are vertical ; 

 and instead of causing the creature to ad- 

 vance they cause it to recede : it is by bend- 

 ing the abdomen suddenly downwards, and 

 bringing it immediately under the sternum, 

 that it strikes the water, and consequently by 

 darting backwards that the animal makes its 

 way through that liquid. From what has now 

 been said it may be imagined that the Crustacea 

 whose conformation is the best adapted for 

 swimming, have the abdomen relatively largely 

 developed, and this is, in fact, what we always 

 observe ; the Amphipoda and Decapoda ma- 

 croura are examples ; whilst, in the walking 

 Crustacea, such as the Crabs, the Caprella, 

 the Oniscus, &c. this portion of the body 

 attains but very insignificant dimensions. 



In the swimming Crustacea the appendages 

 of the penultimate segment of the abdomen 

 also become important organs of locomotion, 

 inasmuch as they for the most part terminate 

 in two broad horizontal plates, which, with 

 the last segment, also become lamelliform, con- 

 stitute an extensive caudal fin arranged in the 

 manner of a fan. 



We have already said that the thoracic ex- 

 tremities alone constitute true ambulatory 

 limbs. When destined for swimming only, 

 their segments are lamelliform, and the palp, 

 as well as the stem, contributes to form the 

 kind of oar which each of them then con- 

 stitutes. The Copepoda supply us with in- 

 stances of thoracic extremities particularly 

 destined for swimming, and a corresponding 

 structure is observed in certain Podophthalmia, 

 such as the Mysis. (See fig. 386.) 



To conclude, this stemmatous portion of the 

 thoracic extremities, whilst it still preserves 

 the general form which we have assigned it, is 

 modified in some cases to serve for walking 

 as well as swimming, or to aid the animal as 

 an instrument for burrowing with facility, and 

 making a cavity for shelter among the sand. 

 Thus in the Decapods that burrow, the last seg- 

 ment of the tarsus assumes a lanceolated form, 

 and in the swimming Erachyura, the same 

 segment, especially of the last pair of extre- 

 mities, appears entirely lamellar. 



We have only further to add that in a great 

 number of species one or several pairs of the 

 thoracic extremities are modified so as to 

 become instruments of prehension; some- 

 times it is the last segment of the limb which, 

 acquiring more than usual mobility, bends in 

 such a manner as to form a hook with the 

 preceding segment ; sometimes it is this penul- 

 timate segment which extends below or by the 

 side of the last, so as to form a kind of im- 

 moveable finger with which it is placed in 

 opposition. In the first instance these instru- 

 ments are denominated subcheliform claws, in 



the second chclte simply, or cheliform claws. 

 We shall revert to these organs when we come 

 to treat of the apparatus of digestion. 

 2. Apparatus of Sensation. 



A. Nervous System. When endeavouring 

 to form as accurate and complete an idea as 

 possible of the tegumenlary skeleton of the 

 Crustacea, we began by studying it in its suc- 

 cessive states of development in the embryo, 

 and then compared the various stages of 

 transition in which it met our observation, 

 with the permanent conditions in which it 

 finally remains in the organic series, classed 

 in conformity with the structural affinities of 

 the different genera. In the study of the 

 nervous system, upon which we are now about 

 to enter, the same mode of proceeding will 

 lead us to analogous results. 



The deep situation of the nervous system, 

 and the transparency of the filaments and 

 various masses which compose it, are each 

 obstacles to its observation until it has arrived 

 at a somewhat advanced stage of development. 

 It was, in fact, only after the sternal canal had 

 begun to appear under the form of an enlarge- 

 ment, edged by a double series of tubercles, 

 which prove to be the rudiments of the motor 

 muscles "of the extremities, that Rathke* was 

 able to catch a sight of the earliest traces of the 

 nervous system in the Astacus fluviatilis, and 

 even this was no more than the portions be- 

 longing to the head and thorax. All that can 

 be seen then amounts to very little ; in the part 

 behind the mouth, eleven pairs of whitish spots 

 are arranged in two longitudinal series perfectly 

 distinct from one another, and situated on either 

 side of the mesial plane. It is otherwise easy 

 to perceive that a pair of these spots corres- 

 ponds to each ring, setting out from, but in- 

 cluding those of the mandibles. Neither the 

 cesophageal cords nor the cephalic ganglions 

 are then distinct. 



At a later period these rudiments of the 

 nervous system undergo remarkable modifica- 

 tions. The six first ganglions of each series 

 approach those that are symmetrical with them 

 severally, so as to become united along the 

 median line, and, at length, to form a simple 

 chain of ganglions corresponding to the six 

 rings, whose appendages are the mandibles 

 and the five pairs of maxillary extremities. 

 The ganglions, on the contrary, which corres- 

 pond to the five posterior thoracic rings, continue 

 to form a double series. During this time the 

 sternal canal is evolved so as to surround the 

 nervous system with a firm and solid sheath. 

 At a period of the incubation still farther ad- 

 vanced, that is to say, during the time which 

 elapses from the birth of the young Crustacean 

 to that at which it attains its full growth, new 

 and important changes take place. First, the 

 four most anterior cesophageal tubercles, in 

 other words, those which correspond to the 

 mandibles, to the jaws, and to the first pair of 

 maxillary limbs, become united, by approach- 

 ing one another along the mesial line, so 



* Uutersurhungen liber die Bildung dcs Fluss- 

 krebses. 



