SKELETONS, SHELTERS AND SPECIAL DEFENCES 641 



cuticle lining the fore- and hindgut, in apodemes and in the eggshell. It 

 appears to be universally present in arthropods and has also been identified 

 in members of certain other phyla, a survey of which is presented in Table 

 15.4. Chitin forms a substantial proportion of the crustacean cuticle, from 

 60-80 % of total organic matter. Protein, representing the remainder of 

 organic matter, is present in two forms, a water-soluble arthropodin, and 

 a water-insoluble sclerotin. The rigidity of the organic skeleton is due 

 largely to the presence of tanned scleroproteins. Some analyses of crusta- 

 cean integuments (organic constituents) are presented in Table 15-5. 

 Arthropodin has been analysed into its amino-acid constituents (30). 



Structurally, chitin is a long-chain polymer. It is similar to cellulose, but 

 with N-acetylglucosamine, linked 1 :4(j8), forming the repeating unit. 



NH.COCH3 



NH.COCH3 CH 2 0H H NH.COCH3 CH 2 0H 



Active systems obviously exist for the synthesis and destruction of chitin, 

 the relative and absolute amounts of which change during the life of the 

 animal. At each moult the cuticle is largely digested, resorbed and formed 

 anew, and chitin in the new cuticle is synthesized from sugars derived 

 ultimately from glycogen reserves of the animal. During the moulting 

 period reserves of protein, lipoid and glycogen in the hepatopancreas are 

 mobilized, both for nutrition of the animal during its fast and for fabrica- 

 tion of new cuticle. Part is transferred to the epidermis, where glucose 

 molecules are aminated and acetylated, leading to the formation of chitin 

 (Fig. 15.2). At the end of the moult, levels of glycogen and glucosamine in 

 the epidermis are greatly reduced (64#, 107, 109, 128). 



Moulting. In crustaceans moulting begins with a loosening of the old 

 cuticle from the underlying epidermal cells. At the same time a fluid is 

 secreted which dissolves part of the old cuticle, and this cuticular material 

 is resorbed. New cuticle — endocuticle and epicuticle — is laid down under- 

 neath the old endocuticle which is to be discarded. The old cuticle splits at 

 certain predetermined lines, which are weakened by the action of moulting 

 fluid, and the animal emerges. The exuvium which is cast off consists of the 

 epicuticle and that portion of the old endocuticle which was not dissolved. 



The cuticle, when first produced, is soft, but in most crustaceans it 

 gradually becomes hardened or sclerotized (decapods, isopods, amphi- 

 pods). This hardening or tanning process produces rigid lateral binding of 

 the protein chains by powerful cross-linkages. The agents responsible for 

 tanning are orthoquinones, which form the lateral linkages between the 

 protein chains. Quinones are produced by oxidation of polyphenols through 

 activity of an enzyme, polyphenol oxidase. Polyphenols themselves are 

 derived from blood tyrosine through the action of tyrosinase, also circulat- 

 ing in the blood. A suggested scheme of events is the following. Some 



M.A.— 21 



