BRACHIOPODS. 



245 



brachiopods are fastened to some marine object by a fleshy peduncle, which passes 

 out between the valves in the centre of the hinge line, or in a corresponding position 

 in those forms where no true hinge is present. 



The inside of the shell is lined by a membrane, which is called the mantle, from 

 its resemblance to a similar structure among the molluscs. Close up to the hinge 

 line is the visceral mass, which is small in proportion to the size of the shell. 

 The mouth is situated in the centre of the visceral mass. The oesophagus communi- 

 cates with the stomach, into which open the ducts from the liver, while the intestine, 

 in most forms, is short, and ends without any external opening, but in others is longer, 

 and terminates in a vent on the right side of the mouth. The alimentary tract is 

 supported in the spacious body cavity by a membrane analogous to the mesentery of 

 the vertebrates. The body cavity is lined with cilia, which keep the contained fluids 

 in constant motion, while prolongations of the cavity extend into the lobes of the 

 mantle, thus forming a rudimentary circulatory system. The body cavity communi- 



FiG. 248 —Anatomy of Waldlmmia, il, arms, m,n, peduncle, p, oesophagus, g, stomach, », liver, », mtestine. 



cates with the exterior by two or four ducts, which in the older works were described 

 as hearts, but it is now known that they are urogenital in function, and should be 

 compared with the segmental organs of worms. 



Tlic nervous system is much better developed than in the Polyzoa, and consists of 

 an oesophageal ring and, in the lower forms, two lateral cords ; in the higher, of a 

 more complex structure. No sense organs are known. The muscles which open and 

 close, the shell and control the other 

 movements of the animal are well 



Pig, 



developed. 



There now remains to be de- 

 scribed, in this hasty sketch of the 

 anatomy of the group, the arms, 

 which in almost all forms are large. 

 These arms, from which the group receives its name (brachiopoda, arm-footed), arise 

 on either side of the mouth, corresponding to the lophophore of the Polyzoan, and, 



. 249.— Development of TerebratMlina ; a, three segment stage; 

 *, attached; c, middle segment folding up to envelop the first; 

 a, e, later stages. 



