Brachiopoda. 79 



inhabiting great oceanic depths, but are now restricted 

 to a few species. They possess shells FU-. 48. 



of two valves, one of which is large, 

 placed ventrally or posterior, and hav- 

 ing a beak pierced with a hole, through 

 which afoot-stalk projects whereby the 

 animal is anchored. The other valve 

 is smaller and dorsal or anterior ; it 

 bears on its inner surface a delicate 

 shelly loop for the attachment of the 

 peculiar arms from which the name of 

 the class is derived. These arms are 

 long, hollow, often spiral processes, face - 

 clothed with tentacles, and their to-and-fro motions 

 cause currents which bring the food within the reach 

 of the mouth of the stationary animals ; they also 

 serve as accessory organs of breathing. The valves 

 of the skull are joined, either by horny matter as in the 

 duck-bill shells (Lingula) of Australia, which have no 

 shelly loop, or by tooth-like hinges, as in the lamp- 

 shells (Terebratula\ and there are several muscles for 

 opening and others for closing the valves. The mantle 

 in Brachiopoda bears bristles on its margin and is full 

 of blood-spaces, which are the chief breathing organs 

 in these animals, and there are usually three hearts 

 for driving on the blood, one chief heart lying on the 

 stomach and two accessory on the mantle wall. The 

 hinge-shelled forms are aproctous. 



The larvae of Brachiopoda are worm-like, locomo- 

 tive, and possess eyes and ear-sacs, but these organs 

 disappear in the fixed adult in which the ciliated 

 head lobe of the embryo becomes converted into the 

 basis of the arms. 



