14 SCHMIDT ON THE COMPARATIVE PHYSIOLOGY 



lived. Oesterlen * subsequently gave a full description and ter- 

 minology. The last-mentioned transparent, and in other respects 

 structureless membrane, with its manifold appendages (teeth, 

 scales, hairs, &c.), forms the innermost layer of the intestinal 

 tube ; upon this lies the reproducing mucous membrane which 

 we have mentioned, and lastly upon the latter, from the pylorus 

 to the anus, layers of transverse and longitudinal smooth mus- 

 cular fibres. Glands, cylindrical epithelium and such like, can- 

 not be detected upon it ; with difficulty we recognise slight in- 

 dications of hexagonal cells, which enable us to determine its 

 mode of development. The whole of this internal apparatus 

 consists of chitine, that peculiar substance which forms the c - 

 taneous system of the same animal, and consequently, of which 

 are composed all those parts which are annually cast oif and 

 must be reproduced. Probably the same holds good with all 

 Crustacea and perhaps with all the Articulata ; I made the ob- 

 servation too late in the year to be able to test its applicability 

 to other families and genera. 



The intestinal canal of the MoUusca, as also their cutaneous 

 system, resembles muscle. There is nothing remarkable in Unio, 

 Helix, LimncBus or Lirnax ; the smooth elements of the layers of 

 longitudinal and transverse fibres are narrower than those of the 

 adductor muscle. The intestine oiAscidia mammillata exhibited 

 the same relations. 



G. Cutaneous System. 



The external coverings of the invertebrate animals exhibit 

 extraordinary variety in their minute structure, as also in their 

 chemical composition. We here meet with phaenomena which 

 no one would a jwiori expect, and which, when combined with 

 others, overthrow any remaining chemico-physiological distinc- 

 tion between animals and plants. We shall consider the che- 

 mical relations according to the great natural orders, which, on 

 the other hand, are characterized by those relations. 



1. Articulata. 

 We cai^not make any use of the older observations in this de- 

 partment ; they were adapted to the existing state of knowledge 

 at that time, but are now merely of historical interest. Odier^s t 



* Miiller's Archiv, 1840, p. 387 et seq. 



\ Mimolre dc la Snciete d'Histoire Naturelle, torn. i. p. 29 et seq. 



