752 CRUSTACEA. 



1. Of the skin or tegumentary skeleton, and animals. It is never found in the properly 



of the organs of locomotion. membranous state, save at the time of the Crus- 



In the definition which has been given of tacea casting their shell; at this period it is 



the Crustacea, one of the most important cha- interposed between the corium and the solid 



racters was derived from the nature and dispo- covering, ready to be cast off, and has the 



sition of their tegumentary system. And it is appearance of a pretty dense and consistent 



from this point that we shall start in laying membrane, in spite of its thinness. It forms, 



before our readers a detailed account of the as among animals higher in the scale, a kind of 



peculiarities of organization presented by this inorganic lamina, applied to the surface of the 



class of animals. By pursuing this course all corium, from which it is an exudation. After 



the subsequent parts of the present article will the fall of the old shell, it becomes thicker and 



appear clearer, the disposition of the internal very considerably firmer, owing to the deposi- 



organs, their forms, their mutual relations, &c. tion or penetration of calcareous molecules 



being in a great number of instances readily within its substance, as well as by the addition 



explicable by the various modes of confer- of new layers to its inner surface. The degree 



mation of the modified skin, which in this of hardness finally acquired, however, and the 



class performs the important office of the in- amount of calcareous matter deposited within 



ternal skeleton among the Vertebrata. it, vary considerably ; in many members of the 



In some Crustacea the skin always con- class it remains semi-corneous, in a condition 

 tinues soft, but in the greater number it very similar to that of the integuments of in- 

 presents a great degree of solidity, and forms sects, with which, moreover, it corresponds 

 a solid casing, within which are included very closely in point of chemical composition; 

 the whole of the soft parts. This difference in the higher Crustaceans, again, its composi- 

 in the condition of the tegumentary envelope tion is very different : thus, whilst chitine in 

 is generally found to coincide with the pre- combination with albumen is the principal 

 sence or absence of particular organs for the element in the tegumentary skeleton of some 

 purposes of respiration ; and in fact it is easy species, this substance scarcely occurs in the 

 to understand that in those species in which proportion of one or two-tenths in the carapace 

 this important function is performed by the of the Decapods, which, on the contrary, con- 

 surface of the body at large, the integument tains sixty and even eighty per cent, of phos- 

 required to be membranous, whilst in those in phate and carbonate of lime, the latter sub- 

 which the covering is of stony hardness, a con- stance particularly occurring in considerably 

 dition which renders it incompetent to expose larger proportions than the former.* 

 the blood to the contact of the atmospheric With regard to the pigmentum, it is less a 

 air dissolved in water, respiration can only be membrane or reticulation than an amorphous 

 performed by the medium of organs especially matter diffused through the outermost layer of 

 contrived and set apart for the purpose. the superficial membrane, being secreted like 



When the tegumentary envelope of the Cms- this by the corium. Alcohol, ether, the acids, 



tacea is studied among the more elevated indi- and water at 21 2 Fahr. change it to a red in 



viduals of the class, it is found to possess a the greater number of species ; but there are 



somewhat complex structure ; parts may be some species in which it may be exposed to the 



distinguished in it comparable to those which action of these different agents without under- 



are known to constitute the integument of the going any perceptible change.f 



Vertebrata. Among the Bracht/ura, for in- The epidermic layer hardened in different 



stance, the integument consists of a corium degrees is the part which mainly constitutes 



and an epidermis with a pigmentary matter of the tegumentary skeleton of the Crustacea. In 



a peculiar nature destined to communicate to its nature it is obviously altogether different 



the latter membrane the various colours with from that of the internal skeleton of the Verte- 



which it is ornamented. brata; still its functions are the same, and this 



The corium or dermis, as among the Verte- physiological resemblance has led naturalists to 



brata, is a thick, spongy, and very vascular mem- speak of these two pieces of organic mecha- 



brane; on its inner surface it is intimately con- nism, so dissimilar in their anatomical rela- 



r.ected with a kind of serous membrane, which tions, under the common name of skeleton. 



lines the parietes of the cavities in the Cms- The tegumentary skeleton of the Crustacea 



tacea in the same manner as the serous mem- consists, like the bony skeleton of the Verte- 



branes line the internal cavities among the Ver- brata, of a great number of distinct pieces, 



tebrata ; these two membranes, divided in the connected together by means of portions of the 



latter order by the interposition of muscular epidermic envelope which have not become 



and bony layers, which cover and protect the hardened, in the same way as among the 



great cavities, become closely united when higher animals certain bones are connected by 



these layers disappear, as they do in the Crus- cartilages, the ossification of which is only 



tacea in consequence of the important changes accomplished in extreme old age. On the 



that take place in the conformation of the ap- varieties which these pieces present in their 

 paratus of locomotion. 



The corium, again, among the Crustacea, is 



completely covered on its outer surface by a * Chevrcul and Geoffrey, Journal Complemcn- 



membranous envelope unfurnished with blood- \*j du BI :? ict n ; de f Sciences Medicalcs, Avril 



vessels, and which must be held in all respects J 8 f p *J' lne Edwards ' Hist " Nat ' dcs Crustaccs, 



as analogous to the epidermis of the higher " t' Lassuigue, Journal d. Fhurmacic, t. vi. p. 174. 



