754 



CRUSTACEA. 



The Loemodipods, the Entomostraca, and the 

 Ilaustellate Crustacea present us with instances 

 of this condition, which calls to mind one of 

 the stages through which the embryo of the 

 higher species, whose development is the most 

 complete, is known to pass. 



Each segment of the body, when it attains its 

 normal condition, consists of two distinct ele- 

 ments : the central or annular portion, and cer- 

 tain appendices which it supports. 



The central or annular portion of the seg- 

 ments of the tegumentary skeleton presents, in 

 its most simple state, the appearance of a com- 

 plete ring, but instead of a single piece it is 

 requisite to count in its composition no fewer 

 than eight, as has been demonstrated by the 

 inquiries of M. Audouin on the structure of the 

 thorax of insects,* inquiries the results of which 

 are immediately and almost wholly applicable 

 to the Crustacea so nearly allied to the insects 

 in their organization. Each ring is divided 

 first into two arcs, the one superior or dorsal, 

 the other inferior or ventral, and each arc may 

 present as many as four elementary pieces. 

 Two of these pieces by being united in the me- 



Fig. 378. 

 t t 



as to appear but one; yet the comparative 

 study of the apparatus in the different members 

 of the class at large, leaves no doubt of their 

 existence severally. 



Fig. 380. 



D 



S 5 



Thorax of an Atelecyclus seen from below. 

 a, sternal pieces of the second thoracic ring ; b, 

 episternal piece of the corresponding ring ; c, epi- 

 meral pieces ; d, apodemata, which run from the 

 sternum to the epimera, and separate the inser- 

 tions of the extremities ; e, antipenultimate ring 

 of the thorax presenting the orifices of the female 

 reproductive organs. 



It frequently happens that the tegumentary 

 membrane is folded so as to penetrate more or 

 less deeply the interior of the ring among the 

 different organs which fill the cavity. These 

 folds, which may become solid laminae by 

 being impregnated with calcareous salts, have 

 received the name of apodemata, and always 

 proceed from the lines of conjunction of the 

 different pieces, or of the different rings with 

 one another. We shall have occasion to revert 

 to this part of our subject very shortly. 



Fig, 381. 



Theoretical figwe illustrating the composition of tlie 



tegumentary skeleton of Crustacea. 

 D, Dorsal arc ; t, t, tergal pieces ; e, e, epimeral 

 pieces ; F, ventral arc ; s, s, sternal and episternal 

 pieces j P, insertion of the extremities. 



dian line constitute the tergum (Jig. 378, D) ; 

 the superior arc is completed on either side by 

 two other pieces, known under the name of fames 

 or epimeral pieces (Jig. 378, e). The inferior 

 arc presents in its composition an exact counter- 

 part of the superior. Two of the four pieces 

 into which it may be resolved constitute the 

 sternum, situated in the median line, and are 

 flanked by the two episternums. The two arcs 

 thus composed, instead of cohering by their 

 edges, leave a space for the insertion of the 

 lateral appendages or extremities which corre- 



Fig. 379. 



e 



Anterior portion of the body of an Amphipoda. 



t, tergum of the fourth thoracic ring ; e, epimera 



of the same ring. 



spond with them. It is true, indeed, that we 

 have no instance of any single ring which exhi- 

 bits the whole of these pieces distinct from one 

 another ; in general several are anchylosed so 



* Annales des Sc. Nat. torn. i. 



Thorax of the Maja Squinado, shewing the apode- 

 mata which form septa between the sternum and 

 the epimeral pieces of the thoracic rings. 



The structure of the ring once investigated 

 in the manner we have done, let us now pro- 

 ceed to inquire in what manner the different 

 rings by the modifications they undergo, and 

 by the divers modes of union they present, give 

 rise to the variety of forms we observe among 

 the Crustaceans. 



By general consent and usage, three regions 

 are recognized in the bodies of these animals, 

 a head, a thorax, and an abdomen; and from this 

 custom we shall not depart, although we must 

 avow that these denominations are only derived 

 from very clumsy views, and are calculated to 

 convey false impressions in regard to the nature 

 and composition of the parts so named, by 

 leading the mind to liken them to the grand 

 divisions entitled head, thorax, and abdomen 

 in the Vertebrata. Nevertheless, with the ex- 

 ception of the objectionable names, the division 

 of the body into three regions is not less a fact 

 as regards the organization of the Crustaceans ; 

 arid the one-and-twenty rings of which, as we 

 have said, their body consists in the type to 

 which every member of the class may be re- 

 ferred, are generally found divided into three 



