CLASSES OF THE MOLLrSCA. 5 



3. The Pteropoda * inhabit the sea only, and swim with a pair 

 of fins, extending outwards from the sides of the head. 



The other mollusca are acephalous, or destitute of any distinct 

 head ; they are all aquatic, and most of them are attached, or 

 have no means of moving from place to place. They are divided 

 into three classes, characterised by modifications in their breath- 

 ing-organ and shell. 



4. The Brachiopoda f are bivalves, having one shell placed on 

 the back of the animal, and the other in front ; they take their 

 name from two long ciliated arms, developed from the sides of 



Figs. 4, 5, 6. BracMopoda.t 



the mouth, with which they create cuiTonts that bring theia 

 food. These arms were formerly supposed to take the place of 

 the feet in the previously-mentioned classes. They are, how- 

 ever, essentially breathing organs, and consequently the term 

 Brachionohrandiia (arm-breathers) has been proposed for the 

 eiToneous one of Brachiopoda (arm-footed). 



5. The Lamellihranchiata, § or ordinary bivalves (like the 

 oyster), breathe by two pairs of gills, in the form of flat mem- 

 branous plates, attached to the mantle ; one valve is applied to 

 the right, the other to the left side of the body. This class is 

 sometimes called Conchifera. 



The Tunicata have no shell, but are protected by an elastic, 

 gelatinous tunic, with two orifices ; the breathing organ takes 

 the form of an inner tunic, or of a riband stretched across the 

 internal cavity. These together with the Polyzoa, and perhaps 



* Pteron, a wing. 



t Brackioti, an arm. 



t Fig. 4. (3.) Rhynchonella psittacea, Chem. sp., dorsal valve, with the animal 

 (after Owen). 5,6, 2'erelratula austrulis. Quay. From specimens collected by Mr. 

 Jukes. (2.) Ideal side view of both valves (/, the cardinal muscles, by which the valves 

 ore opened). (1.) Dorsal valve. These woodcuts have beeu kindly lent by Mr. J. E. 

 Gray. 



^ Lamellibranchiata, plate-gilled. 



