OF THE INVERTEBRATE ANIMALS. 13 



the principle of the greatest possible extent of s\;rfacc, numerous 

 anastomosing canals containing the formative fluid run. The 

 latter belong to the vascular system, whilst the former belong to 

 the cutaneous system, of the chemical composition of which they 

 partake. This relation is naturally most striking where the con- 

 trast of the cutaneous system in general is most distinctly per- 

 ceptible, as in the Articulata. The tracheal system of insects, as 

 also that of the tracheal spiders, the respiratory sacs of the pul- 

 monary spiders and the gills of the Crustacea, consist of Chiline 

 according to investigations made on the cockchafer, the common 

 house-fly and Ateuchus sacer among insects, the craw-fish and 

 crabs among Crustacea, and Phalanyium (parietim/m) and Epeira 

 {diadema) as tracheal and pulmonaiy spiders. This Chitine is 

 a peculiar substance resembling woody fibre but containing ni- 

 trogen and forming the cutaneous skeleton of these animals, 

 further details of which will be given when treating of the latter. 

 Its insolubility in potash, even after continued boiling, is highly 

 characteristic of this substance ; the organs under consideration 

 may thus be easily isolated and prepared for microscopic analy- 

 sis. The chitinous tissue does not exhibit the least change, and 

 the elegant ramifications of the tracheae especially may thus be 

 exquisitely separated and examined. 



F. Organs of Digestion. 



The substance of the alimentary canal, the other tube which 

 is in direct communication with the external world, appears to 

 belong to the cutaneous system. This conclusion is based upon 

 the examination of the stomach of the Craw-fish. It consists of 

 an external, thin, transparent, difficultly separated mucous mem- 

 brane, and an internal transparent membrane which unites the 

 separate parts of the complicated framework of the stomach, and 

 is covered with hairs of various forms. The latter membrane is 

 cast annually, the former produces the new stomach, or rather 

 the new epithelium. 



Von Bar *, with his usual acuteness of observation and clear- 

 ness of description, first examined it, and at the same time re- 

 futed numerous fables regarding the change of the stomach of 

 the Craw-fish which had been current since the period at which 

 Van Helmontt and Geoffroy J (the younger of the two elder) 



• ^\n\\tr'% ArcMv, 1834, p. 510 et seq. f Lithiasis, cap. vii. 



I Memoire de V Academie des Sciences, 1 709, p. 309. 



