SKELETONS, SHELTERS AND SPECIAL DEFENCES 



651 



Chemical analyses are available for the shells of Nautilus, Spirula and 

 Sepia. Organic matter is small in amount, around 2-5 %, somewhat higher 

 values occurring in cuttle-bone than in the hard shell of Nautilus. Chitin 

 occurs in the shell or endoskeleton of all groups (Table 15.4). The mineral 

 matter is predominantly aragonite (94-99-7%) (23). 



Brachiopods. The shells of brachiopods, which superficially are so 

 similar to those of lamellibranchs, are likewise bivalved. The two valves 

 are dorsal and ventral; the posterior end of the ventral valve projects 

 beyond the dorsal, and bears a notch or aperture through which a stalk 

 protrudes, attaching the animal to the substratum. Brachiopods are 

 divided into two groups with the following skeletal characteristics — 



1. Ecardines. Shells usually soft (chitinous), only lightly strengthened 



Beak 



Dorsal 

 valve 



Foramen 



Cardinal 

 process. 



Tooth 

 socket 



Impressions 

 of adduction 

 muscles 



Shell J 



loop 



Fig. 15.7. Valves of a Brachiopod Magellania fiaveseens. (After Davidson) 



{Left) Entire shell, viewed dorsally; (right) interior of dorsal valve showing shelly 

 loops (skeletal supports). 



with lime salts (except Crania). A hinge and internal supports for the arms 

 are lacking. 



2. Testicardines. Shells heavily charged with calcareous spicules. The 

 valves are hinged and there is usually an internal skeleton supporting the 

 arms (Fig. 15.7). 



Most brachiopods are attached by a stalk or pedicle to some rock or 

 other support. In some species, e.g. Crania, the ventral valve is firmly 

 attached to its support. Lingula possesses a long free stalk and lives in a 

 burrow in sandy bottom. In testicardinate brachiopods there are two lateral 

 teeth at the posterior margin of the ventral valve, and these fit into cor- 

 responding sockets in the dorsal valve so as to form a hinge. An endo- 

 skeleton is present in the form of plate-like processes or spiral lamellae, 

 which project into the shell cavity and support various parts of the soft 

 structures. 



The valves of ecardines, with the exception of Crania, contain a high 

 proportion of organic matter, from 25-40% dry weight {Lingula, Discinisca, 



