756 



CRUSTACEA. 



fectly distinct, and differ but little from one 

 another; and the seven abdominal rings, in 

 like manner, remain moveable, and only differ 

 from those of the thorax as they do from one 

 another by a relatively inferior degree of deve- 

 lopment. In the majority of the Isopods the 

 structure of the tegumentary skeleton is essen- 

 tially the same as in the Amphipods ; but there 

 occurs a greater inequality of development be- 

 tween the thoracic and the abdominal rings, 

 most of the latter remaining more or less in a 

 rudimentary state. 



In the Apus and the Nebalia we conti- 

 nue to find the rings of the thoracic and abdo- 

 minal portions of the tegumentary skeleton 

 nearly equal in size and similar in form; but 

 the cephalic section, instead of presenting the 

 same conformation as these two portions of the 

 body, constitutes superiorly an immense shield, 

 which extends over the rings of the thorax and 

 conceals them. This dorsal shield or buckler, 

 which is denominated Carapace by zoologists, 

 also occurs among the whole of the Podoph- 

 thalmians, and more than all besides conspires 

 to give to these animals their distinguishing 

 peculiarities of shape. Inquiries, of which it 

 would be tedious to give a detailed account in 

 this place, have led us to discover that the 

 carapace of these Crustaceans is neither more 

 nor less than the superior arc of the third or 

 fourth cephalic ring, enormously developed, 

 and which in attaining its large dimensions 

 laps over and modifies the conformation of a 

 greater or smaller number of the neighbouring 

 rings.* 



In the generality of the Stomapods the cara- 

 pace does not quite cover and conceal the two 

 first cephalic rings, which indeed continue dis- 

 tinct and moveable ; but in the whole of the 

 Decapods these rings cohere with one ano- 

 ther and with the following ones, and unite 

 more and more intimately under the carapace, 

 which then covers the whole of the head as well 

 as the thorax. In the Macroura the anterior 

 extremity of the carapace only extends over the 

 ophthalmic or first cephalic ring ; but in the 

 Brachyura it bends around this ring so as to 

 include it, and to go to unite underneath with 

 the next segment. As we ascend in the series 

 of Crustaceans, we observe the carapace en- 

 croaching more and more upon the thorax. 

 In the Squills the three last cephalic and three 

 first thoracic rings are nearly lost by becoming 

 blended with those to which the carapace be- 

 longs ; they scarcely retain any mobility, and 

 protected above by this shield, unite intimately, 

 and remain imperfect in their tergal portions ; 

 the four last rings of the thorax continue, on the 

 contrary, free, and are in almost every particular 

 similar to those of the abdomen. In the Mysis 

 this union of the cephalic shield with the seg- 

 ments of the thoracic division of the tegumen- 

 tary skeleton is carried further, for there are 

 not more than two of these rings which remain 

 distinct. But it is in the Decapods that the 

 carapace attains its greatest development, and 



* See my Hist. Nat. des Crustaces, t. i. p. 23. 



that its influence upon the evolution of the 

 thoracic segments is carried the farthest. 



In these animals the framework of the body 

 does not appear at first sight to consist of more 

 than two portions, the one anterior, formed by 

 the carapace, and representing the cephalic and 

 thoracic segments conjoined ; the other poste- 

 rior, formed by the abdomen. In reality, the 

 first fourteen rings of the body are covered by 

 this enormous buckler, and are so intimately 

 conjoined as to have lost all their mobility ; the 

 whole of the thoracic segments thus hidden 

 below the carapace, are connected with it in 

 their superior part, they are only joined with 

 one another underneath an'd laterally ; and their 

 tergal parts having, in consequence of this, be- 

 come useless, are no longer to be found, being 

 in some sort replaced by the great cephalic 

 buckler; thus the whole of these rings, in con- 

 formity with this arrangement, are imperfect 

 and open above. 



Hitherto we have not been able to deter- 

 mine whether the carapace of the Podophthal- 

 mia is formed at the expense of the third or 

 of the fourth ring of the tegumentary skeleton ; 

 but we have the strongest reasons to conclude 

 that this buckler is neither more nor less than 

 the dorsal arc of one or other of these cephalic 

 rings, and not of the two conjointly. In fact 

 we can here demonstrate a composition analo- 

 gous to that which we have already pointed 

 out as characteristic of every arc, whether supe- 

 rior or inferior, of the different rings in their 

 state of complete development, to wit, a tergal 

 portion and two lateral or epimeral pieces. In 

 following the embryo of the River-crab in its 

 progressive stages of development, Rathke* 

 observed the carapace to be formed of three 

 pieces, which at length became consolidated 

 so as to form but one. In many of the Deca- 

 pods it is even easy to perceive this structure 

 or composition in the carapace of adults, inas- 

 much as there exist lines marking the conjunc- 

 tion, and accurately indicating the respective 

 limits of the different pieces of which this great 

 dorsal plate is composed. 



The general form of the carapace depends in 

 great measure on the relative development of 

 these different pieces; in the Macroura the 

 tergal portion of the carapace extends but a 

 short way backwards, whilst the lateral or 

 epimeral pieces reach as far as the begin- 

 ning of the abdomen, and being no longer 

 kept at a distance by the tergum, meet in the 

 median line of the back, and are there con- 

 joined. In the Brachyura, on the contrary, the 

 tergal portion is that which is especially deve- 

 loped, so that it constitutes the whole of the 

 upper part of the carapace, whilst the lateral 

 pieces, thrust outwards and underneath, only 

 form a narrow band above the bases of the ex- 

 tremities. 



It is also in consequence of modifications 

 analogous to those on which the existence of the 

 carapace depends, that in other Crustacea the 



* Untersuclmngen ucber die Bildung dcs Fluss- 

 krebses, &c. Tr. iu Aunales des Sciences Nat. t. 20. 



