762 



CRUSTACEA. 



1 . Of the skin or tegument ary skeleton, and 

 of the organs of locomotion. 



In the definition which has been given of 

 the Crustacea, one of the most important cha- 

 racters was derived from the nature and dispo- 

 sition of their tegumentaiy system. And it is 

 from this point that we shall start in laying 

 before our readers a detailed account of the 

 peculiarities of organization presented by this 

 class of animals. By pursuing this course all 

 the subsequent parts of the present article will 

 appear clearer, the disposition of the internal 

 organs, their forms, their mutual relations, &c. 

 being in a great, number of instances readily 

 explicable by the various modes of confor- 

 mation of the modified skin, which in this 

 class performs the important office of the in- 

 ternal skeleton among the Vertebrata. 



In some Crustacea the skin always con- 

 tinues soft, but in the greater number it 

 presents a great degree of solidity, and forms 

 a solid casing, within which are included 

 the whole of the soft parts. This difference 

 in the condition of the tegumentary envelope 

 is generally found to coincide with the pre- 

 sence or absence of particular organs for the 

 purposes of respiration ; and in fact it is easy 

 to understand that in those species in which 

 this important function is performed by the 

 surface of the body at large, the integument 

 required to be membranous, whilst in those in 

 which the covering is of stony hardness, a con- 

 dition which renders it incompetent to expose 

 the blood to the contact of the atmospheric 

 air dissolved in water, respiration can only be 

 performed by the medium of organs especially 

 contrived and set apart for the purpose. 



When the tegumentary envelope of the Crus- 

 tacea is studied among the more elevated indi- 

 viduals of the class, it is found to possess a 

 somewhat complex structure; parts may be 

 distinguished in it comparable to those which 

 are known to constitute the integument of the 

 Vertebrata. Among the Brachyura, for in- 

 stance, the integument consists of a corium 

 and an epidirmis with a pigmentary matter of 

 a peculiar nature destined to communicate to 

 the latter membrane the various colours with 

 which it is ornamented. 



The corium or dermis, as among the Verte- 

 brata, is a thick, spongy, and very vascular mem- 

 brane; on its inner surface it is intimately con- 

 nected with a kind of serous membrane, which 

 lines the parietes of the cavities in the Crus- 

 tacea in the same manner as the serous mem- 

 branes line the internal cavities among the Ver- 

 tebrata ; these two membranes, divided in the 

 latter order by the interposition of muscular 

 and bony layers, which cover and protect the 

 great cavities, become closely united when 

 these layers disappear, as they do in the Crus- 

 tacea in consequence of the important changes 

 that take place in the conformation of the ap- 

 paratus of locomotion. 



The corium, again, among the Crustacea, is 

 completely covered on its outer surface by a 

 membranous envelope unfurnished with blood- 

 vessels, and which must be held in all respects 

 as analogous to the epidermis of the higher 



animals. It is never found in the properly 

 membranous state, save at the time of the Crus- 

 tacea casting their shell; at this period it is 

 interposed between the corium and the solid 

 covering, ready to be cast off, and has the 

 appearance of a pretty dense and consistent 

 membrane, in spite of its thinness. It forms, 

 as among animals higher in the scale, a kind of 

 inorganic lamina, applied to the surface of the 

 corium, from which it is an exudation. After 

 the fall of the old shell, it becomes thicker and 

 very considerably firmer, owing to the deposi- 

 tion or penetration of calcareous molecules 

 within its substance, as well as by the addition 

 of new layers to its inner surface. The degree 

 of hardness finally acquired, however, and the 

 amount of calcareous matter deposited within 

 it, vary considerably; in many members of the 

 class it remains semi-corneous, in a condition 

 very similar to that of the integuments of in- 

 sects, with which, moreover, it corresponds 

 very closely in point of chemical composition ; 

 in the higher Crustaceans, again, its composi- 

 tion is very different: thus, whilst chitine in 

 combination with albumen is the principal 

 element in the tegumentary skeleton of some 

 species, this substance scarcely occurs in the 

 proportion of one or two-tenths in the carapace 

 of the Decapods, which, on the contrary, con- 

 tains sixty and even eighty per cent, of phos- 

 phate and carbonate of lime, the latter sub- 

 stance particularly occurring in considerably 

 larger proportions than the former.* 



With regard to the pigmentum, it is less a 

 membrane or reticulation than an amorphous 

 matter diffused through the outermost layer of 

 the superficial membrane, being secreted like 

 this by the corium. Alcohol, ether, the acids, 

 and water at212Fahr. change it to a red in 

 the greater number of species; but there are 

 some species in which it may be exposed to the 

 action of these different agents without under- 

 going any perceptible change.f 



The epidermic layer hardened in different 

 degrees is the part which mainly constitutes 

 the tegwnentary skeleton of the Crustacea. In 

 its nature it is obviously altogether different 

 from that of the internal skeleton of the Verte- 

 brata ; still its functions are the same, and this 

 physiological resemblance has led naturalists to 

 speak of these two pieces of organic mecha- 

 nism, so dissimilar in their anatomical rela- 

 tions, under the common name of skeleton. 



The tegumentary skeleton of the Crustacea 

 consists, like the bony skeleton of the Verte- 

 brata, of a great number of distinct pieces, 

 connected together by means of portions of the 

 epidermic envelope which have not become 

 hardened, in the same way as among the 

 higher animals certain bones are connected by 

 cartilages, the ossification of which is only 

 accomplished in extreme old age. On the 

 varieties which these pieces present in their 



* Chevreul and Geoffrey, Journal Complemen- 

 taire du Diction, des Sciences Medicales, Avril 

 1820. Milne Edwards, Hist. Nat. des Crustaccs, 

 t. i. p. 10. 



t Lassaigne, Journal d. Pharmacie, t. vi. p. 174. 



