BRACHIOPODA. 



575 



living ; but no special distinctive importance can be attached to these 

 facts, for similar statements may be made of almost all the great 

 groups, representatives of which are known from the earliest 

 formations. 



The body is enclosed in a bivalve shell, of which one valve is 

 called dorsal, and the other ventral (Fig. 457). Both valves lie upon 

 corresponding folds of the integument called the mantle lobes, and 

 are often connected posteriorly by a kind of hinge, beyond which the 

 usually more arched ventral valve projects like a beak (Fig. 458, St). 

 The ventral valve is usually larger than the dorsal, and is either 



St 



Fio. 458. Anatomy of Waldhclmia australis, side view of partly dissected specimen (after 

 Hancock). Ar arms (the proximal loop of the left arm has been cut away) ; Do dorsal 

 valve ; L liver ; Ma adductor, Md divaricator muscles ; the liver ducts of the left side cut 

 short; Oe oesophagus; St peduncle; Tr folded margins of the funnel of the right nephridium 

 (oviduct) ; Ve ventral valve ; Vvj anterior wall of body. 



directly fused with foreign bodies, or the animal is attached by a 

 peduncle projecting through the opening in the beak. The peduncle 

 may however pass out between the two valves (lAngula). In Crania, 

 which is attached by its ventral valve, the peduncle is absent. It is 

 doubtful which surface of the body the stalk belongs to ; according 

 to the view stated below, which is admittedly highly speculative, 

 it is a projection of the ventral surface of the body. The valves 

 of the shell are cuticular structures secreted by the skin and 

 impregnated with calcareous salts; they are not opened by a 

 ligament, but by special groups of muscles (Fig. 458, Md} ; they 

 are also closed by muscles, which are placed near the hinge and pass 



