200 



ZOOLOGY 



mantle fold is less conspicuous because it is hidden deep in 

 the spire, but the edge of it may be seen protruding from the 

 margin of the aperture. In the naked moUusks — e.g. the 

 slugs — the mantle appears as a reduced cap on the back of 

 the animal. 



Appendages are not present in most mollusks ; locomotion 

 is effected in the snail by the whole ventral part of the body 



Fig. 19.3. — The snail Helix aspersa, fully extended from the right. After 

 Howes, a, anus at edge of mantle ; ph. the breathing pore ; s, shell ; 

 p, edge of " aperture " of shell ; ga. the genital aperture ; ty. one of the two 

 tentacles carrying the eyes ; t, anterior tentacles ; /o, upper lip. Fi'om Lang. 



— the foot. In these animals a great mucus gland lies just 

 above the skin of the foot and pours out, from near the anterior 

 part of the bodj^, a stream of mucus over which the animal 

 glides. The different modifications that the foot shows in 

 different mollusks are considered in the following chapters. 

 In contrast to the foot the head bears very characteristic 

 appendages — the tentacles, which can be rolled out of 

 harm's way or be stretched to an extraordinary distance. 



Organs of Nutrition. — The different species of mollusks feed 

 on almost every kind of organic stuff, from the fine organic 

 debris suspended in water to the succulent green leaves of 



