^^ 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 Ray Soc. London, 1851. This work is rich la 

 anatomical details, many of which are new. 



Vail Be7ieden. Recherches sur quelques Crustaees inferieurs, in the 

 Ann. d. Sc. Nat. XVI. 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 conti'actility, 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 

 horny substance, was first discovered with the 

 Insecta by Odier (Mem. d. 1. Soc. d'Hist. Nat. de 

 I'aris, I. 182:3, p. 29). Latterly, it has been care- 

 fully investigated by C. Schmidt (Zur vergleich. 

 I'hysiol. d. wirbellos. Thiere, 18-15, p. 32), who 

 found, moreover, that the cutaneous skeleton of 

 Crustacea has the same composition as that of 

 Insecta. 



* [ § 264, note 1.] Of the results of Lavalle^s 

 ■ihservations it may be well to add his concluding 

 remarks ; after a minute description of each por- 

 tion of the tegumentary apparatus, he concludes : 

 " I shall here only observe further, that my inves- 

 tigations seem to be in complete opposition to the 

 theories which make the shell of the Crustacea an- 

 alogous (homologous) to the scaly epidermis of 

 Serpents and Lizards. I see no analogy (ho- 

 mology) between the shedding of the shell of the 

 Crustacea, — which divests them of organs des- 



1 We are indebted to Valentin for the re- 

 searches, few as they are, which have hitherto 

 been made on the internal structure of the skele- 

 ton of Crustacea ; see his Piepertor. f. Anat. u. 

 Physiol. I. 1836, p. 122. 



Liivalle is about to publish microscopical re- 

 searches on the structure of the cutaneous skeleton 

 of the Decapoda: see Ann. d. Sc. Nat. 1847, p. 

 352.* 



lined to give the body its form and volume, to 

 serve as points of attachment to the locomotor 

 muscles, to furnish the instruments of prehension 

 and mastication ; organs placed not only on the 

 surface of the body, but often immersed in the 

 midst of soft parts, and in which we find an organ- 

 ization such as I have described, — and the peri- 

 odical shedding observed in reptiles of a thin 

 epidermis, without consistency, completely unor- 

 ganized and incapable of fulfilling any of the 

 uses to which the shell ia destined. My re- 



