SKELETONS, SHELTERS AND SPECIAL DEFENCES 639 



pod Temora, euphausiid Thysanoessa) consists almost entirely of calcium 

 phosphate. 



The relative proportions of mineral constituents in the skeleton vary in 

 different regions of the animal, and with age. The latter factor is brought 

 out in the analyses from Clarke and Wheeler (23) for Homarus shown in 

 Table 15.3. As the animal ages, CaC0 3 decreases relative to other con- 



TABLE 15.3 



Mean Analyses of Lobster Shells 

 (percentage ash) 



stituents and there is a marked relative increase of Ca 3 (P0 4 ) 2 . Slight rela- 

 tive increases of CaS0 4 and MgC0 3 are also evident. 



Organic Composition of Crustacean Exoskeleton. The cuticle is 

 formed by the underlying epidermis (hypodermis) and comprises two 



Outer epicuticle 



Inner epicuticle 



Pigmented zone 



of endocuticle 



Duct of 



tegumental 



gland 



Calcified zone 

 of endocuticle 



Non-calcified 

 zone of, 

 endocuticle 



Epidermis 



Fig. 15.1. Diagrammatic Representation of the Fully Formed Cuticle of 

 a Decapod Crustacean, as seen in Cross-section. (From Dennell (27).) 



regions — a relatively thick inner layer containing chitin, the endocuticle; 

 and a thin outer layer, the epicuticle, containing protein but lacking chitin 

 (Fig. 15.1). An extensive treatment of the arthropod cuticle is given by 

 Richards (109). 



Chitin present in the endocuticle is bound with protein in a protein- 

 chitin complex. Chitin occurs not only in the body wall, but also in the 



