758 



CRUSTACEA. 



Fig. 385. 



ment, or that the forms they assume, in har- 

 mony with the uses they serve, render 

 them liable to be mistaken. It is very dif- 

 ferent in the embryo; here, in fact, as among: 

 the simplest forms of the series, the whole 

 of the extremities are at first similar; and 

 it is only in consequence of ulterior develop- 

 ments that each pair finally assumes diver- 

 sities of form and character in relation with 

 the various functions to which they are espe- 

 cially destined. 



In its most perfect state of development, 

 the extremity in the Crustacean consists of three 

 principal parts : the stem (), which is the most 



Fig. 386. 



Thorax of the Astacus Fluviatilis, showing the dis- 

 position of the apodemata and the thoracic cells. 



of the latter. This rule, which has been dis- 

 cussed by M. Geoffrey St. Hilaire under the 

 title of la loi de balaucement organique, or law 

 of organic equivalents, is found to apply in the 

 present instance ; for the Crustacea in which 

 the cephalic portion of the tegumentary skele- 

 ton is developed in the greatest degree, (viz. 

 the Brachyura) present the abdominal portion fe 

 of the body of very small dimensions ; whilst, 

 on the other hand, in the Macroura, or those 

 species in which the abdominal portion of 

 the body arrives at its maximum of develop- 

 ment, and performs a very important office in 

 the business of locomotion, the cephalic por- 

 tion is relatively greatly inferior in size. 



With regard to its disposition the abdomen 

 is simple enough ; the rings of which it con- 

 sists are in general moveable upon one 

 another, and even when they are consolidated, 

 present no apodemata projecting from their 

 interior. It is also deserving of remark that 

 the elementary pieces of the different rings are 

 not very distinct, and sometimes even appear 

 to be partially wanting. 



Let us now go on to examine the portion 

 of the tegumentary skeleton belonging to the 

 extremities or that portion of the external 

 skeleton of the Crustacea which may be re- 

 garded as an appendage to the more essen- 

 tial covering of the head, thorax, and ab- 

 domen. 



The Crustacea present this invariable cha- 

 racter, that the whole of the appendages belong 

 exclusively to the inferior arc of their tegu- 

 mentary rings, a point in which they resemble 

 the Arachnidans, and differ like these from 

 Insects, in which one or two of the thoracic 

 rings generally present a pair of extremities 

 supported by the superior arcs, as in the An- 

 nelidans, in which the dorsal segment of each 

 of the rings almost always carries a pair of 

 extremities fashioned in the same manner as 

 those belonging to the ventral arcs.* We 

 have already said that a pair of appendages 

 ought to be found attached to each ring ; but 

 it very frequently happens that many of the 

 pairs are completely checked in their develop- 



* Vide Annelida, p. 167. 



essential and most constant part, formed of a 

 variable number of articulations ; the palp (6), 

 an appendage which is detached from one of the 

 three first articulations of the stem, but almost 

 always from the first; and the whip (fouetj (c), 

 which is sent oft above and to the outer side of 

 the palp. It but rarely happens, however, that 

 these three organs exist simultaneously ; occa- 

 sionally not more than one of them can be 

 demonstrated ; and sometimes the whole three 

 are altogether wanting. 



Fig. 387. 



First cephalic ring of the Squilla separated from 

 the rest of the head, and bearing one of the 

 ocular peduncles. 



The first ring presents no appendages except 

 in the very highest Crustaceans, and even then 

 they are simple in their composition, and never 

 exhibit more than the stem, which arises from 

 a more remote check to their development 

 dating from about the commencement of their 

 embryonic evolution; these are the ocular pe- 

 duncles. 



The second and third pairs of extremities 

 constitute the antenna. These are wanting in 

 a certain number of the inferior species, and 

 even in those among which they occur, they 

 vary considerably in their structure : they may 

 for instance present one only, or two, or the 

 whole of the three elements of which we have 

 spoken. But as the three first pairs of ap- 



