On the nature and permeability of chitin. I.--The chitin lining the foregut of decapod crustacea and the function of the tegumental glands

Abstract
Chitin is widely distributed throughout the animal kingdom and also occurs in the Fungi. Wester (1910) has shown that it is present in all Arthropods, invariably in the exoskeleton and lining the respiratory systems and except in some Arachnids, in the greater part of the gut. In the Mollusca it occurs in the jaws and radulæ, in the shell and sometimes in the gut of Cephalopods and sometimes in the shell and lining of siphons in Lamellibranchs. The setæ of Annelids are of chitin and occasionally, as inLumbricusandAphrodile, the gut is lined with it. Chitin is also present in the shell, peduncle and spines of some Brachiopods,e. g., Lingula,in some Plyzoa, in the Hydroids and very occasionally, as in the gemmules ofSpongilla,in the porifera. According to Wester, it never occurs in protoza, Echinodrema, worms other than Annelids or in vertebrates. Wherever it occur, chitin gives the same chemical reactions (Zander, 1897 ; Wester, 1910) and has the same physical properties, e. g., specific gravity and refractive index (Sollas, 1907 ; Becking and Chamberlin, 1925) and specific rotation which is always lævorotatory (Irvine, 1909). It is surprising that so little is known about the properties of this very important substance. In the Arthropods especially, where it covers the entire surface of the body and where respiration and in some cases absorption in the gut-indeed practically all interchange between the interior of the body and the external medium-must take place through it, exact knowledge of the permeability of chitin and of the condition which control this is clearly essential for a full understanding of the life of the animal. It was the almost complete absence of knowledge on this subject which led to the initiation of this series of researches. Ideal material for this purpose was found in the uncalcified chitin which lines the foregut (“œesphagus” and “stomach”) of the Decapod Crustacea, of which relatively large pieces can be obtained from animals of average size. The lobster,Homarus vulgaris, owing to the relatively large size of the foregut, provided the great bulk of the research material, other large Decapods, such aPalinurus vulgaris,Cancer pagurus, Maia squinadoandCarcinus mœnas, being used for comparative purposes.