FOKM OF BODY OF MOLLUSCA. 321 



two respiratory tubes in the opposite region of the body. They are 

 continued into special divisions of the mantle-cavity, owing to the 

 division of the latter into an upper smaller, and a lower larger 

 cavity, which are separated by a partition. The water is brought to 

 the lower one by the efferent tube, and passes through the gills ; 

 streaming through the orifices in them, into the branchial plates 

 or the intrabranchial cavity, and so into the upper division of the 

 mantle-cavity, into which the anus also opens. 



The edge of the mantle is often the seat of special differentia- 

 tions, which are generally of the form of tentacular processes, and 

 are sometimes of a fair size. 



The second differentiation in the body of the Lamellibranchiata 

 affects the ventral surface, which is differentiated even in the 

 Placophora ; it becomes flattened, and serves as the organ by which 

 the animal creeps along. It is formed by the development of a 

 muscular foot, which is more or less separated from the rest of the 

 body (Fig. 167, Ap), and which can be protruded from the cleft in 

 the mantle, sometimes to a considerable extent. It is then hatchet 

 or club-shaped, and functions as a locomotor organ. The two lateral 

 surfaces of the foot are ordinarily produced into a median edge, but 

 in some it is flat and sole-shaped, as in Chitons. 



Many Lamellibranchs live under conditions in which this organ is 

 not required, and it is then atrophied, as in the fixed Oysters and 

 Anomise, and Scallops ; in the latter, locomotion is effected by the 

 action of the mantle and its shells. 



The Scaphopoda are forms allied to the Lamellibranchiata, but 

 intermediate between them and the Gastropoda. The body, which 

 is enclosed by a shell, is provided with a mantle-cavity, from which a 

 trifid foot can be protruded. A part which carries the mouth is head- 

 like in form, but is really more of a proboscis, for it does not contain 

 the nerve-centres, and is, moreover, enclosed in the mantle-cavity. 



§ 250. 



The velum is largest in the Gastropoda and the thecosomatous 

 Pteropoda, and is absent in those only in which the earliest larval 

 stages are not free (Land Snails). It has the form of a large, and 

 frequently symmetrically lobate, organ (Fig. 170, A B G v), which in 

 some is retained for a longer time, and so enables the body to continue 

 swimming about (Macgillivraya). The development of this organ, 

 which in its lower stages is merely represented by a circlet of cilia, 

 appears to be correlated with the development of a shell, for when 

 this is developed the cilia are less widely distributed. The cephalic 

 portion of the body is alone free; and it compensates for the absence 

 of other locomotor organs by the great development of its cilia, and 

 of its ciliated margin. The velum increases in size, and undergoes 

 great complications of form, in proportion to the increase in the 

 weight of the body due to its shell. 



