It has been shown (Yonge, 1932) that the integument of the Decapod Crustacea, as exemplified by the uncalcified lining of the foregut of the lobster,
Homarus vulgaris
, consists of two layers which differ widely in nature and origin. There is a thin superficial cuticle which is hyaline, possesses adsorbed lipin, and is formed by the widely distributed tegumental glands the function of which had previously been obscure. The actual chemical nature of this thin layer was not determined but it is
not
chitin from which it can be distinguished by a variety of chemical and physical tests. The underlying and much thicker layer of the integument consists of lamellated chitin formed by the cells of the epithelium. The present research was designed to determine in broad outline the permeability of this membranous integument, and in particular the influence upon this of the bounding cuticle and the general biological significance of the cuticle. In the Crustacea, Jordan and Lam (1918) found that the foregut and hindgut of
Astacus
, which are lined with chitin, behave as semipeimeable membranes, allowing water, but not dissolved substances, either electrolytes or non-electrolytes, to pass through under the influence of osmotic pressure. Similar results were obtained by Yonge (1924) with the foregut of
Nephrops
. Very different results were obtained from similar experiments with the midgut of both
Astacus
and
Nephrops
, indicating that the peculiar properties of the remainder of the gut are due to the chitinous lining. Murlin (1902) and Nicholls (1931) have shown that the chitin which lines the so-called midgut in
Oniscus, Porcellio
, and other land Isopoda, and in
Ligia oceanica
respectively, is permeable to the end-products of digestion. Krogh (1915) states that the gills of
Astacus
are practically impermeable to urethane. Fischel (1908), Koehring (1930, 1931), Gickelhorn (1931), and Bond (1933) have all found some evidence for the penetration of the integument of various Cladocera and Copepoda by vital stains.