649 



long muscular stalk, called the peduncle (Tig. 2). This larger valve is there- 

 fore often called the *' peduncle valve " ; by English writers it is called the 

 " ventral valve," although in the natural position of stalked forms it is always 

 the uppermost. The smaller valve is called the "brachial" or "dorsal" 

 valve. In microscopic structure, also, the shell differs from that of the 

 Mollusca (Fig. 3). The two valves may be united by a hinge at the hinder, 

 or umbonal end (ARTICULATA, Fig. 4), or they may not (INA&TICULATA, 

 Fig. 5). 



The shell-valves are lined by two mantle-folds, or extensions of the body- 

 walls, which contain prolongations of the body-cavity ; and in them the 

 generative products are formed. The free margins of the mantle-folds are 

 beset with bristles (setae, shown at 6. in Fig. 2). Muscles pass across the body 

 of the animal from one valve to the other; they serve to open and to close 

 the valves, and to move them sideways. The imprints of these muscles 

 on the shell are important in the study of fossil brachiopods (Fig, 5). 



fig, *. A HINOED BRACHIOPOD (Ra^nesquina alternatd). 

 ~ The upper figure shows the hinge from outside; the 

 lower figures show the inside of the valves, p.v., 

 peduncular or ventral valve; ~b.v., brachial or dorsal 

 valve ; h., hinge-line ; d., deltidium ; /., foramen ; a., 

 area; i.a. ando.a., its inner and outer portions; t., 

 tooth; d.s., socket in which tooth works; c.p., car- 

 dinal process ; m.c., muscle scars. Two-thirds natural 

 size. 



The viscera (Fig. 6) lie near the hinder umbonal part of the shell, and the 

 mouth is directed towards the fore-part or opening of the shell. Around 

 the mouth is a somewhat horseshoe-shaped disc, bearing ciliated tentacles, 

 and called the lophophore (tuft-bearer). It is often produced into two 

 processes or "arms," which fill the fore-part and sides of the shell-cavity, 

 and are often spirally coiled. From these "arms" is derived the name 

 '* Brachiopoda " (fyaxiu*, an arm, and *6vs, a foot) ; they are often sup- 

 ported by a calcareous skeleton, the shape of which is of great import- 

 ance in classifying fossil forms. The movement of the cilia of the lopho- 

 phore serves to drive to the mouth currents of water containing food- 

 particles. A lophophore is also found in the Bryozoa, and in a few other 

 sessile marine animals (Phoronis, Bhaldopleura, Cephalodiscus). The mouth 



