THE BRACHIOPODA. 403 



the ventral valve of the shell being the posterior lobe. And 

 the region of the anterior wall of the pallial cavity which lies 

 behind or below the mouth will answer to the neural aspect 

 of the Polyzoon. 



On the other hand, if the segments of the body of the 

 larval Brachiopod are true somites, and the discoidal surface 

 of the hindermost corresponds with the similarly formed end 

 of the larva of Lacimilarla^ as Prof. Morse suggests, the 

 dorsal lobe of the mantle will, as before, represent part of 

 the haemal surface of the body, but the ventral lobe will be- 

 long to its neural surface — and can no longer properly be 

 termed mantle, but will rather answer to the foot of one of 

 the higher Mollusca. 



The Brachiopoda are distinguishable into two groups, 

 the Articulata and the Liarticulata. In the Articulata, the 

 two valves are united by a hinge, and the ventral valve is 

 usually provided with teeth, which are received in sockets 

 of the dorsal valve. The gullet ascends in the middle line 

 toward the dorsal valve, and the intestine descends toward the 

 opposite, or ventral, valve, and there ends in a caecum. The 

 dorsal valve often gives rise to spiral or looped shelly pro- 

 cesses to w^hich the arms are attached.-^' The valves are 

 brought together by a pair of adductor muscles, which pass 

 directly from valve to valve ; and they are separated by di- 

 varicator muscles, which run obliquely from the ventral valve 

 to a median process (the cardinal process) of the hinge-line 

 of the dorsal valve. The impressions of the attachments of 

 these muscles on the inner surfaces of the valves have con- 

 siderable systematic importance. Very often the ventral 

 valve is produced into a sort of spout, through which passes 

 the peduncle by which the animal is attached to rocks. At 

 the sides of the visceral chamber the thickened edge of the 

 dorsal lobe of the mantle passes into that of the ventral lobe. 



The substance of the shell is very often traversed by 

 numerous canals perpendicular to its surface, w^hich contain 

 prolongations of the mantle.^ 



This division contains the families of (1) The Terehra- 

 tulidce, (2) the Spiriferidm, (3) the RhynchonelUdm, (4) the 

 Orthidw, and (5) the Productidce, of which the second, fourth, 

 and fifth are extinct and almost wholly palaeozoic, no species 



' The structure of the shell has beeii particularlv studied by Carpenter. 

 ("Reports of the British Association," 1844-' 47. and Introduction to David- 

 son's " Fossil Brachiopoda.") See also King, *' Trans. Royal Irish Academv," 

 1869. ^^ J . 



