CLASSES OF THE MOLLUSCA. 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 + are bivalves, haying 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. Brachiopoda.t 
the mouth, with which they create currents that bring them 
food. ‘These arms were formerly supposed to take the place of 
the feet in the previously-mentioned classes. They are, how- 
eyer, essentially breathing organs, and consequently the term 
Brachionobranchia (arm-breathers) has been proposed for the 
erroneous one of Brachiopoda (arm-footed). 
&. The Lamellibranchiata, § 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 Twnicata have no shell, but are protected by an elastic, 
gelatinous tunic, with two orifices; the breathing organ takes 
the form of an inner funic, or of a riband stretched across the 
internal cavity. These together with the Polyzoa, and perhaps 
* Pteron, a wing. 
t+ Brachion, an arm. 
t Fig. 4. (3.) Rhynchonella psittacea, Chem. sp., dorsal valve, with the animal 
(after Owen). 5,6, Terebratula australis, Quoy. From specimens collected by Mr. 
Jukes. (2.) Ideal side view of both valves (f, the cardinal muscles, by which the valves 
are opened). (1.) Dorsal valve. These woodcuts have been kindly lent by Mr. J. EB. 
Gray. 
§ Lamellibranchiata, plate-gilled. 
