SECT. XI. 



MOLLUSC A. 



231 



bends it into an elbow ; then having fixed the hooked 

 point firmly in the sand, by a sudden contraction 

 of the muscles it springs 

 to a considerable height 

 and distance, and leaps 

 actively along the sur- 

 face of the sand. The 

 lowest part of fig. 166 is a 

 magnified section of the 

 foot, showing the muscular 

 system which gives the ani- 

 mal that power. It con- 

 sists of many rows of longi- 

 tudinal muscles, interlaced 

 at regular distances by 

 transverse fibres. When 

 the foot is extended, the 

 Cockle has the power of 

 distending it by filling a 

 network of capillary tubes 

 with water till it is almost 

 transparent. The water is 

 also distributed through 

 the body and into the gill- 

 chamber, which opens and 

 shuts every ten minutes or 

 oftener, in order to main- 

 tain the supply ; and it has 

 egress through the pores 

 in the mantle and foot, for 



, . ' , Fig. 166. Foot of Cockle. 



some burrowing mollusks 



squirt it out through the foot when disturbed. This 

 water-system is unconnected with the circulation of 

 the blood. 



Each bivalve mollusk is both male and female ; and 

 the fertilized eggs pass into the gills of the parent, 



