124 



MOVEMENT 



parasites, move by the wriggling of their body, and their ad- 

 vance in the right direction is aided by the fact that their 

 cuticle is ringed. A longitudinal section of the skin resembles 



Fig. 42. Planaria polychroa swimming, magnified about 4. 1. Eye. 2. Cili- 

 ated slit at side of head. 3. Mouth of proboscis. 4. Outline of the pharjoix 

 sheath into which the pharynx can be withdrawn. 5. Reproductive pore. 



the teeth of a saw, and the projecting ridges enable them to 

 move forward as their body sways from side to side. 



The great group of molluscs is so diverse that it presents 

 a great variety of modes of movement. Many of them swim. 



Fig. 43. Helix pomatia, the edible snail. Side view of shell and animal expanded 

 and creeping and gliding along. From Hatschek and Cori, old types. 1. Mouth. 

 2. Anterior tentacles. 3. Eye tentacles. 4. Edge of mantle. 5. Respiratory 

 pore. 6. Anus. 7. Apex of shell. 8. Foot. 9. Reproductive aperture. 



Pecten, the shell of which used to be worn by pilgrims, swims 

 by means of a curious jerky motion, alternately opening and 

 shutting the valves of its shell. Some molluscs move by 

 gliding. The snail, for instance, secretes a mucus from a gland 

 which traverses the foot and opens just under the mouth, and 

 the cilia which cover the lower surface of the snail's foot 

 "row" in this mucus and the whole animal slowly glides 



