638 THE BIOLOGY OF MARINE ANIMALS 



the ratio 35:65) (103). The tubes and burrows of polychaetes are described 

 in later sections. 



Crustacea. Arthropods, as a group, are characterized by a chitinous 

 cuticle. This is usually stout, but at intervals on the trunk and limbs it 

 becomes flexible so as to form joints. The cuticle is an external armour 

 protecting soft parts and moulding body form. It provides attachment for 

 muscles and a skeleton for development of limbs, jaws and other mouth 

 parts. 



The organic constituents of the cuticle are chitin and proteins, and often 

 it is further hardened by the deposition of lime salts. These give the cuticle 

 a stony hardness in certain ostracods, barnacles and crabs. Periodically, 

 during the growth of the animal, the hard outer layers of the cuticle are 

 separated from the inner layers, ruptured and shed in a moult or ecdysis. 

 The soft inner layers then expand to accommodate the body. Expansion is 

 effected by osmotic changes in the blood and by absorption of sea water 

 (see Chapter 2). In Uca, for example, the water content of the body in- 

 creases by about 16% immediately after ecdysis. Moulting processes are 

 regulated by temperature ; in temperate and cold regions the frequency of 

 ecdysis is highest in summer, and declines or ceases in winter months 

 (49, 127). 



Many groups of crustaceans are provided with a shell or carapace. This 

 is a dorsal fold of skin and cuticle which arises from the hind border of 

 the head and extends for a greater or lesser distance over the trunk. 

 Ostracods possess a bivalved carapace covering the whole body ; in Mala- 

 costraca it covers the thorax. The carapace has disappeared in isopods and 

 amphipods. An internal skeleton is sometimes present as ingrowths of the 

 cuticle. These structures, known as apodemes, serve for insertions of 

 muscles; in decapods they unite to form a framework, the endophragmal 

 skeleton. 



On the basis of skeletal composition, crustaceans fall into two main 

 categories — namely barnacles with predominantly calcareous shells, and 

 softer-bodied forms, shrimps, lobsters, etc., the skeletons of which contain 

 much organic matter. The shells of barnacles (Lepas, Balanus) contain a 

 minor fraction of organic matter (2-5 % dry weight). CaC0 3 is the major 

 inorganic constituent, forming 96-99% of mineral matter; MgC0 3 is 

 present in amounts up to 2-5 % (Table 15.1). 



The proportion of organic matter is high in the shrimp skeleton, 77 % 

 (dry weight) in Crangon \ in Homarus it forms 38%; and is low in crab, 

 29% in Eriphia. Absolute values of organic and inorganic constituents 

 show much variation with age and species. CaC0 3 predominates among 

 mineral constituents. It is low in shrimps, 60% ash or less; lobsters and 

 crayfish show values around 74-78%. Calcium phosphate (Ca 3 (P0 4 ) 2 ) is 

 present in notable quantities (11% mineral matter in Homarus, and 

 27-49% in shrimps Crangon and Pandalus). Magnesium is also a con- 

 spicuous constituent, MgC0 3 forming 8-15% of skeletal ash in various 

 decapods. Inorganic material in the skeletons of minute crustaceans (cope- 



