SKELETONS, SHELTERS AND SPECIAL DEFENCES 



643 



hormonal stimulus releases the activity of tyrosinase, and blood tyrosine is 

 converted into hydroxyphenol. The polyphenols diffuse outwards to the 

 epicuticle, where they are oxidized to orthoquinones; the latter harden 

 the epicuticle and outer layers of the endocuticle. The tegumental glands 

 appear to be the source of polyphenol oxidase (10, 11, 27, 68, 69). 



Another important, and sometimes dominant, factor in hardening is the 

 deposition of lime salts. After moult, calcification of the cuticle takes place 

 rapidly. In Cancer and Carcinus the outer layer of the cuticle becomes 

 calcified in 1-2 days, followed by deeper lamellae. The degree of calcifica- 

 tion stands in inverse relationship to the amount of protein present in the 

 cuticle. In newly formed cuticle prior to calcification, and in species with 



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4- 



2- 



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Hepa.topa.ncrea.s^ 



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Epidermis 



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 CO 



i — i — r 

 A-*- B 

 Moult 



Feeding 



D 



Formation of 

 new Cuticle 



Fig. 15.2. Variations in Amount of Glycogen Contained in the Hypodermts 

 and Hepatopancreas of Cancer pagums during Course of an Intermoult 



- non-feeding period; feeding period. Moult at A. A-B, period of absorption 



of water immediately after moult and of calcification. B-C, water absorption and calci- 

 fication complete; period of hardening of integument. C-D, completion of integument, 

 feeding and tissue growth. D-A, premoult, new integument forming. (From Renaud 

 (107).) 



non-calcified cuticle, there are roughly equivalent quantities of chitin and 

 protein. With the progress of calcification the quantity of protein added to 

 the cuticle decreases. Calcification thus replaces a large proportion of 

 protein that would otherwise be needed for hardening the cuticle, and is an 

 economical process in an environment rich in calcium. 



Prior to the moult, calcium is resorbed from the cuticle. The fate of this 

 calcium varies in different species. Calcium is stored in the hepatopancreas 

 (crabs, spiny lobsters), in gastroliths (lobsters), sternal plates and pleopods 

 (isopods), and held in the haemolymph (Fig. 15.3). A certain proportion is 

 also excreted by marine decapods. As the new cuticle hardens, calcium is 

 reabsorbed from the environment and mobilized from stores in gastroliths, 

 hepatopancreas and elsewhere. Blood-calcium concentrations vary in cor- 

 relation with these changes in the cuticle and may rise to five times normal 



