10 



MOLLUSCA. 



The Mollusca are unjointed, unsegmented animals, without jointed 

 appendages. The body is covered by a soft slimy skin. They lack 

 both an internal and external locomotory skeleton, and appear there- 

 fore especially suited for life in water. But few of them are 

 terrestrial, and when this is the case the locomotion is always limited 

 and slow ; while the aquatic forms, in correspondence with the far 

 more favourable conditions for locomotion presented by water, may 

 be endowed with the power of rapid swimming. 



The dermal muscular system plays an important part in the 

 locomotion of these animals, especially that part of it placed on the 

 lower, i.e., ventral, surface of the body. In this region it is greatly 

 developed, and gives lise to a more or less projecting locomotory 

 organ of very various shape, the foot (figs. 492 and 493). The 

 foot always consists of an unpaired median structure, which is some- 

 times divided into several 

 parts and may possess 

 in addition lateral paired 

 portions, the epipodia. 

 Above the foot there very 

 generally exists on the 

 body a shield - shaped 

 thickening of the integu- 

 ment, the so-called mantle, 

 the edges of which, in 

 more advanced develop- 

 ment, grow over the body 

 as a fold of the skin and 

 partially or completely cover it. The surface of this fold of skin 

 secretes calcareous and pigmentary substances, and gives rise to the 

 variously shaped and coloured shells which contain and protect the 

 soft body. In addition to the foot and mantle, the body generally 

 possesses in the anterior region, on either side of the mouth, a pair 

 of lobe-like appendages, the buccal lobes, which are the remnants of 

 a largely-developed larval structure, known as the velum. 



In the higher Mollusca (Cephalophora) the anterior part of the 

 body bearing the buccal lobes, and containing the central parts of 

 the nervous system and the sense organs, is more or less sharply 

 marked off as a head. The part of the body behind the head 

 constitutes the main mass of the animal. Its dorsal portion (the 

 visceral sac) contains the viscera and is frequently spirally twisted, as 

 a result of which the bilateral symmetry undergoes externally a. 



FIG. 493. Lnrva of Vermehts (after La caze-Dnthiers) . 

 S, velum ; Br, gill ; F, tentacle ; P, foot; Oc, eje. 



