496 



MOLLUSKS : ACEPHALS. 



Fig- 477- 



They are not uncommon on the coast 

 of the United States. 



Boltenia pedunculata, 



M.-Edw. One eighteenth. 



New Zealand. 



. This Family contains Simple Ascidians 

 which are fixed, and solitary or gregarious 



The Genus Ascidia contains A. rustica, Linn, found in 

 clusters adhering to stones and floating timbers in Boston 

 Harbor. 



The larvae of Ascidians are tadpole-like in shape, and 

 move freely about in the water. It is claimed that they are 

 related to the Vertebrates in their mode of development, in 

 the relative position of their nervous system, and in possess 

 ing a structure closely like the chorda, dorsalis of verte- 

 brated animals. And according to Darwin, the Vertebrates 

 may have been derived from animals resembling the larva 

 of our present Ascidians I 



SUB-SECTION III. 



THE ORDER OF BRACHIOPODA, OR BRACHIOPODS.* 



This Order comprises mollusks whose shells, com- 

 posed of two valves, occupy a dorsal and ventral posi- 

 tion in relation to the animal. These valves, though 

 unequal in size, are symmetrical in shape. The dorsal 

 valve is the smaller, and is always free and imperfo- 

 rate. The larger valve is the ventral, and has a promi- 

 nent beak through which the organ of adhesion passes, 

 by which the animal is attached to submarine bodies. 

 The two valves are articulated by two curved teeth de- 

 veloped from the margin of the ventral valve, and re- 

 ceived by sockets in the dorsal, and this makes a hinge 

 so complete that the two valves cannot be separated 

 F5 g . 47 8. without injury. 



Brachiopods take their name from two 

 long ciliated arms, shown in Fig. 478, 

 growing from the sides of the mouth, by 

 which they create currents in the wa- 

 ter, and thus secure their food. These 

 animals are found hanging from the un- 

 der sides of shelving rocks, from coral 



* Now regarded as a class. Professor E. S. Morse, however, regards 

 Brachiopods as true Articulates, and closely related to Vermes or Worms. 



