496 



MOLLUSKS : ACEPHALS. 



Fig. 477 . 



Boltenia fedunciilata, 



M.-d\v. One eighteenth. 



New Zealand. 



They are not uncommon on the coast 

 of the United States. 



ASCIDIAD.E. This Family contains 

 Simple Ascidians which are fixed, and 

 solitary or gregarious. 



The Genus Ascidia contains A. rus- 

 iica, Linn., found in clusters adhering 

 to stones and floating timbers in Bos- 

 ton Harbor. It is from the size of a 

 pea to a half or three quarters of an 

 inch in diameter. 



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 

 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 



Fig. 478. 



