108 BRACH1OPODA. 



extends into the mantle as a series of spaces or sinuses ; 

 these produce slight depressions on the interior of the 

 valves, and can often be traced in casts of the interior 

 of fossil specimens as ridges (fig. 40). The body-cavity is 

 filled with a fluid which is kept in motion by means of 

 cilia. A heart appears to be absent, but a vesicle on the 

 dorsal surface of the stomach has been described as 

 such. 



The brachiopods are never colonial animals. Repro- 

 duction takes place sexually, never by budding or fission. 

 The genital organs are placed in the body-cavity, in 

 the sinuses of the mantle, or in both. Some forms are 

 hermaphrodite, others diceceous. 



The greater part of the pallial chamber is occupied by 

 two long processes, the arms (fig. 29, d), given off from 

 the sides of the mouth. These are covered with cirri (h), 

 and they, with the mantle serve as organs of respiration. 

 The cirri also produce a current of water, which carries 

 food to the mouth. 



Of the two valves of the brachiopod, the ventral is 

 always larger than the dorsal, and it is produced into a 

 beak or umbone, the termination of which is generally 

 perforated by a foramen (fig. 32 A, /). Each valve is 

 equilateral, that is to say, a line drawn from the umbone 

 to the opposite margin of the shell divides the valves into 

 two equal and similar parts. This character, combined 

 with the inequality in the size of the valves, renders it 

 easy to distinguish the shell of a brachiopod from that of 

 a lamellibranch. In many forms the two valves are joined 

 together by means of a hinge, these constitute the group 

 Articulata; in others they are held together by muscles 

 only, these form the Inarticulata. The hinge consists of 

 two short curved processes or teeth given off from the 



