CRUSTACEA. 



755 



Talitra Saltator magnified. 



a, head; b, thorax composed of seven distinct 

 rings ; c, abdomen composed also of seven dis- 

 tinct rings. 



equal series of seven, each of which may be 

 held as corresponding with one of the three 

 regions. This law of composition is observed 

 to obtain not only among the more simple 

 species, where the rings generally resem- 

 ble each other most closely, but its influence 

 may be remarked among the most complicated 

 also, and amidst exceptions and contradictions 

 in appearance the most obvious. The head or 

 cephalic region includes the principal organs 

 of sense as among the Vertebrata, the com- 

 mencement of the apparatus subservient to 

 digestion, and the appendages destined to seize 

 and masticate the food. The thorax, strictly 

 speaking, forms no cavity distinct from the pre- 

 ceding, but is its continuation ; the part espe- 

 cially designated thorax, however, is that which 

 is included from front to back between the 

 head and the beginning of the abdomen, and is 

 formed by the rings to which the extremities 

 serving for locomotion are attached. This mid- 

 dle portion of the general cavity of the body 

 contains almost the whole of the viscera. As 

 to the abdomen, it succeeds the last of the 

 thoracic rings, distinguishable by the presence 

 in it of the orifices of the male organs of gene- 

 ration ; the appendices attached to it do not 

 commonly attain any considerable size, and do 

 not serve in a general way as organs of locomo- 

 tion ; to conclude, nothing is found in its inte- 

 rior save muscles and the terminal portion of the 

 intestinal canal, the anal orifice of which exists 

 in the last of the abdominal series of rings. 



These three portions of the tegumentary ske- 

 leton are not always equally distinct, and their 

 respective limits may even vary, for we occa- 

 sionally observe two or three of the foremost 

 thoracic rings detaching themselves, as it were, 

 from this region to which they properly belong, 

 to join or blend with the cephalic rings ; and 

 the same thing may be said in regard to the 

 segments of which each of the remaining divi- 

 sions of the body consists; we in fact know of 

 no specimen of a Crustacean in which the whole 

 of the rings are moveable upon one another ; a 

 certain number of them always appear to be- 

 come consolidated, and this union is frequently 

 so intimate that all traces of its existence are 

 obliterated, so that the section of the body 

 which results from this aggregation of rings 

 appears to consist of no more than a single 



piece, and on a cursory view might be held to 

 be constituted by a simple ring. The shape 

 and size of these compound rings varies also, 

 circumstances which evidently depend on the 

 unequal development of the different pieces of 

 which they severally consist. 



This consolidation of the rings occurs with 

 increasing frequency as we rise in the scale of 

 Crustaceans, and approach those the organiza- 

 tion of which is most complex ; yet there are a 

 considerable number of species which form ex- 

 ceptions to this rule. The consolidation of the 

 rings also shows a tendency to take place in 

 the same order in which the different segments 

 of the tegumentary skeleton appear in the em- 

 bryo, that is to say from before backwards : 

 thus it is generally complete as regards the 

 cephalic rings ; it is more frequent as regards 

 the foremost than the hindmost thoracic rings ; 

 and it but rarely occurs among the abdominal 

 rings. 



The differences which present themselves in 

 the dimensions and forms of the different rings 

 of the tegumentary skeleton, and which concur 

 so essentially in producing varieties in the ge- 

 neral form of the Crustaceans, also show a ten- 

 dency to become greater and greater as we as- 

 cend in the series of these animals, and com- 

 monly influence the cephalic rings in a degree 

 greater than those of the divisions situated 

 more posteriorly. 



To conclude, it is also among the most ele- 

 vated Crustaceans that the tegumentary skeleton 

 is complicated in the greatest degree by the 

 evolution of apodemata in the interior of the 

 rings ; and further, it is in the cephalo-thoracic 

 portion of the skeleton only that these lamina; 

 are encountered. 



A few examples will render these general 

 rules more readily appreciated. 



In the earlier periods of evolution of the 

 embryo of the river-crab, the whole of the rings, 

 which are even then apparent, are of the same 

 focm and dimensions, and the segments, which 

 only appear at a later date, are at first similar 

 to what these rings were in the beginning. 

 This state of uniformity in the composition 

 of the whole of the constituent rings of the 

 tegumentary skeleton, which is invariably tran- 

 sient in the embryo, is not observed as a 

 permanent feature in any perfectly developed 

 Crustacean ; still there are several of these ani- 

 mals which are but little removed from it. In 

 the Branchipods, for instance, the body consists 

 of a long series of rings, having, with the ex- 

 ception of the very first, as nearly as possible 

 the same form and the same dimensions. In the 

 Amphipods (fig. 382) the want of resemblance 

 between the different rings of the body becomes 

 much more remarkable: the first seven become 

 so completely united that they form a single 

 piece, in which no trace even of the lines of 

 consolidation remains, and the conical segment 

 which constitutes the head grows much more 

 slowly than the rest of the body, so that the re- 

 lative dimensions become smaller and smaller 

 as regards the head in proportion as the animal 

 approaches the adult age. The seven rings of 

 the thorax, on the other hand, continue per- 



