^"^ 263, 264. THE CRUSTACEA. 309 



by Ray Soc. London, 1850. In this work are embodied the results of his 

 earlier researches published in the Trans, of the Berwick Nat. Club, &c. 



Darwin. A monograph of the sub-class Cirripedia with figures of all 

 the species. Published by Kay Soc. London, 1851. This work is rich in 

 anatomical details, many of which are new. 



Van Beneden. Eecherches sur quelques Crustaces inferieurs, in the 

 Ana. d. So. Nat. XVL p. 71. — Ed. 



CHAPTER I. 



EXTERNAL ENVELOPE AND CUTANEOUS SKELETON. 



§ 263. 



The External envelope of the Crustacea is more or less solid, and has the 

 form of a multi-articulated, cutaneous skeleton, sometimes of a leathery or 

 horny consistence, but generally consists of a hard, calcareous shell. 



It has, consequently, no contractility, and participates in the movements 

 of the body only in a passive manner, that is, by the interarticular soft 

 skin, and by the antennal and foot-like processes. 



In this cutaneous skeleton, whether it is leathery, horny, or calcareous, 

 there is a peculiar organic substance as its base. This substance, which 

 is found in the cutaneous skeleton of other Arthropoda also, has received 

 the name of Chitine. It resembles cellulose of plants in its insolubility in 

 caustic potass, but differs essentially from it in containing nitrogen. ^^^ 



§ 264. 



Nothing in general can be said as to the Histological composition of this 

 cutaneous skeleton of the Crustacea, for it differs widely not only in the 

 various orders and families, but even in the different parts of the body of 

 the same species. ^^^ Whether hard or soft, it is usually composed of 



1 This Chitine which was formerly taken for a 1 We are indebted to Valentin for the re- 

 horny substance, was first discovered witli tlie searches, few as they are, wliich have hitherto 

 liisecta by Odier (Mem. d. I. Soc. d'Hist. Nat. de been made on the internal structure of the skele- 

 I'aris, I. 1823, p. 29). Latterly, it has been care- ton of Crustacea ; see his Repertor. f. Anat. u. 

 fully investigated by C. Schmidt (Zur vergleich. Physiol. I. 1836, p. 122. 



Physiol, d. wirbellos. Thiere, 1845, p. 32), who Lavalle is about to publish microscopical re- 

 found, moreover, that the cutaneous skeleton of searches on the structure of the cutaneous skeleton 

 Crustacea has the same composition as that of of the Decapoda; see Ann. d. Sc. Nat. 1847, p. 

 Insecta. 352.* 



* [ § 264, note 1.] Of the results of Lavalle^s lined to give the body its form and volume, to 



observations it may be well to add his concluding serve as points of attachment to the locomotor 



remarks ; after a minute description of each por- muscles, to furnish the instruments of prehension 



tion of the tegumentary apparatus, he concludes : and mastication ; organs placed not only on the 



" I shall here only observe further, that my inves- surface of the body, but often immersed in the 



ligations seem to be in complete opposition to the midst of soft parts, and in which we find an organ- 



Iheories which make the shell of the Crustacea an- ization such as I have described, — and the peri- 



alogous (homologous) to the scaly epidermis of odical shedding observed in reptiles of a thin 



Serpents and Lizards. I see no analogy (ho- epidermis, without consistency, completely unor 



mology) between the shedding of the sheU of the ganized and incapable of fulfilling any of the 



Crustacea, — which divests them of organs des- uses to which the shell is destined. My re- 



