55 



ACEPHALA LAMELLIBRANCHIATA. 



Were the test of an Ascidia to be converted into hard 

 shell, symmetrically divided into two plates connected to- 

 gether dorsally by cartilage, and capable of separation so as 

 to expose the mantle along a ventral mesial line, whilst the 

 orifices protruded at one extremity, it would present the 

 closest similarity with many bivalve shell-fish. We pass 

 by a very natural transition from the Ascidians to the La- 

 mellibranchiate Ace2^hala. 



This great section of the headless mollusks is so styled, 

 because of the peculiar arrangement of the respiratory 

 organs in the creatures composing it. The branchial leaf- 

 lets are four in number, usually forming expanded laminse, 

 arranged in pairs on each side of the main mass of viscera. 

 If the number, as in a few species, appear fewer than four, 

 it is so by habitual suppression ; if more, by reduplication. 



This peculiar respiratory apparatus is included within the 

 mantle, but quite free from it. The mantle secretes and is 

 protected by a bivalve shell, the two valves of which are 

 applied to the two sides of the animal. These valves are 

 almost always moveably articulated together at their dorsal 

 edges by a more or less complicated hinge, connected by a 

 more or less developed ligament, and are held close by power- 

 ful adductor muscles passing from the inner surface of one 

 valve to that of the opposite, either one or two in number. 

 The edges of the lobes of the mantle are more or less united, 

 and in certain genera and families are free. Its extreme 

 margin, in a great many, is prolonged in the shape of two 



