FORM OF BODY OF MOLLUSCA. 



319 



secondary and variously-differentiated axes — the dorso- ventral and 

 the transverse. The body is therefore of the original eudipleural 

 form, which is the dominant one in the Vermes and Arthropoda. 



These relations are different in the Gastropoda, where the dorsal 

 cup-like shell gradually encloses the greater part of the body, and 

 leaves a small portion only of the surface of the body exposed in 

 addition to the head and foot. So that while in the previous case 

 the shell was adapted to the body, in this case the soft parts of the 

 body are adapted to the single shell. The body, therefore, becomes 

 asymmetrical, and the aboral pole no longer carries the anus, which 

 becomes lateral in position in consequence of the flexure of the 

 enteron ; this flexure is due to the formation of the shell. All of 

 the many variations from the symmetrical ground-form, which are 

 seen in the Gastropod-body, may be regarded as due to this. 



The primitive similarity in the form of the body, due to the 

 possession of a shell, undergoes great modifications even among the 

 Gastropoda; the Veliger stage is not always developed, and has 

 never yet been observed in the Cephalopoda. But even in this class 

 the form of the body, and the disposition of its viscera, may be seen 

 to be, in all forms, due to the possession of a shell. 



§ 249. 



The velum has different functions in different divisions. In the 

 Lamellibranchiata, where it functions for some time as a locomotor 

 organ, but where it is never independent and soon atrophied, its 

 function is not very great. This may, perhaps, be correlated with 

 the rudimentary character of the future head, and this, again, with 

 the rapid disappearance of the free mode of life in this division 

 (Acephala). 



Two folds, however, which are given off laterally from the dorsal 

 surface, become considerably developed and form a mantle; they 

 surround the body, and excrete 



the shell, which corresponds A , B 



with the lamellas of the mantle 

 in form and size. 



A space, which functions 

 as the respiratory cavity, is 

 developed between the edges 

 of the mantle ; branchiae are 

 developed from the body- wall, 

 and project into it (Fig. 167, 

 A br). In a few Lamelli- 

 branchs (Asiphonia) this en- 

 trance into the mantle- cavity 

 is a cleft of some size, by which water passes in and out, 

 and so carries in nutriment and removes excreta. In most, the 

 two edges of the mantle grow together, and so shut off, more 

 or less completely, the cavity which surrounds the gills, and cause 



Diagram of the relations of the 

 foot and mantle, as seen in transverse sec- 

 tion. A In Lamellibranchiata. B In Cepha- 

 lophora. m Mantle, p Foot, hr Branchiae. 



