132 OYSTERS, AND ALL ABOUT THEM. 



Having thus drawn attention to the superficial contrast 

 between the classes, I proceed to describe the Brachiopoda, 

 which are far inferior to the Conchifera, and bear a stronger 

 relation to the Tunicata and Polyzoa than they do to the 

 other class under our notice. 



BRACHIOPODA. 



GENERAL CHARACTERS. --The animals of this class 

 are distinguished from the Lamellibranchiate Mollusca by 

 the absence of any special branchial apparatus, the respira- 

 tory function being performed by the mantle, which is 

 traversed by numerous blood-vessels. They are also 

 characterized by the possession of a pair of long, ciliated, 

 and usually spiral arms, the analogues of the labial tentacles 

 of the ordinary bivalves. The action of the cilia with 

 which these are clothed produces a current in the water, 

 that carries the particles of food to the mouth, which is 

 situated close to their base. From the presence of these 

 organs the class receives the term Brachiopoda. 



The same class is also known bv the term Pallio- 



i 



branchiata. The valves, instead of being placed on each 

 side of the body of the animal, are situated above and 

 below it; so that they are called dorsal and ventral, instead 

 of right and left, valves. The ventral valve is usually larger 

 than the dorsal, and projects beyond it at the beak, where 

 it is generally perforated to allo\v the passage of a muscular 

 or tendinous peduncle, by which the animal attaches itself. 

 In some cases the peduncle is wanting, and the shell 

 is then fixed by the beak. The connexion of the valves is 

 effected by a pair of teeth, springing from the ventral 

 valve, and locking into corresponding cavities in the dorsal 

 valve. There is no ligament, and the valves of the shell 

 are opened and closed by appropriate muscles. 



