VI. BEACHIOPODA. 327 



the posterior parts remaining in contact. At this part, except in 

 the Ecardines, a hinge is developed just in front of the posterior 

 margin, consisting of projections (teeth) in the ventral valve which 

 fit into corresponding grooves in the dorsal. Opening and closing 

 the valves are, contrary to what occurs in Lamellibranchs, active 

 processes, accomplished by appropriate divaricator and adductor 

 muscles (fig. 299). These produce scars on the shell, important in 

 the study of fossil forms. 



The usually spirally coiled arms, which lie right and left of the 

 mouth and which give the name to the class, fill most of the shell. 

 On the outer side of the spiral axis runs a longitudinal groove 

 which reaches to the tip of the arms and is bounded by a row of 

 small tentacles. By means of cilia on tentacles and groove food is 

 brought to the mouth. These arms strongly resemble the lopho- 

 phore of a phylactolaemate Polyzoan, which only needs extension 

 and coiling to produce this condition. In development the arms 

 of the Brachiopod pass through a lophophore stage. 



In the body there is a body cavity which extends into both, 

 mantle folds. It encloses alimentary tract, gonads, and liver, and 

 is divided into right and left halves by a dorsal mesentery support- 

 ing the intestine. Each half in turn is divided by incomplete 

 septa into anterior, middle, and posterior divisions recalling those 

 of Sagitta (p. 296). If the arrangement of the septa is not so 

 clear as in that form, it is to be explained by the shortening of the 

 long axis and the twisting of the alimentary tract. This latter 

 consists of oesophagus, stomach, which receives the liver ducts, and 

 intestine, which in some species terminates blindly. 



The gonads are chiefly in the mantle lobes. The sexual cells 

 pass outwards through the nephridia, which begin in one coelomic 

 pouch with a wide nophrostome, perforate the septum, and open 

 to the exterior in the next somite. Since usually there are two 

 septa, two pairs of nephridia may occur, but one is usually degen- 

 erate. The nervous system consists of an cesophageal ring with 

 weak dorsal ganglion, which sends nerves into the arms, and a 

 stronger ventral mass representing the ventral chain. The heart 

 lies dorsal to the stomach. 



In development the brachiopods recall both Sagitta and the Annelida. 

 They resemble Sagitta in that in Argiope the coelom arises by out- 

 growths from the archenteron, divided by septa into three pairs of pouches. 

 They are annelid-like in the form of larva and in the presence of chastae 

 which are formed in separate follicles. In an earlier period of the earth 

 brachiopods were so numerous in species and individuals that they are 

 among the most important fossils in the determination of geologic horizons. 



