BRA CHWPODS. 



245 



hracliiopods are fastened to some marine oliject l>y a flesliy pedunele, whieh passes 

 out between the val\ es in tiie centre of the hinge line, or in a coin'sponding position 

 in those forms where no true hinge is present. 



The inside of tiie shell is lined by a membrane, whieh is t'alled the mantle, from 

 its resembhince 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. Tlie alimentary tract is 

 su])ported 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 circul.-itury system. The body ca\ity communi- 



Vnatomy of naldham, 



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. 



The 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 kin)wn. The niuseles which open and 

 close the shell and control the other 

 movements of the animal are well 

 developed. 



There now remains to be de- 

 scribed, in this hasty sketch of the 

 anatomy of the group, the arms, 

 which m almost all forms are large. 

 These arms, from which the grouj) receives its nai 

 on either side of the mouth, corresponding to the 





Fig. 249.— Development of Terebratulina : a, three segment stage; 

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

 (I, (', later stages. 



(brae 



jphop 



iida, arm-footed), arise 

 of the Foly/.o.an, and. 



