MAEINE MOLLUSCS 195 



cxlialent or dorsal siphon. These currents are maintained by the 

 vibratile action of thousands of minute cilia belonging to cells that 

 line the cavities of the body, and serve to supply the animal with 

 both air and food ; for lamellibrauchs, being gill-breathers, derive 

 the oxygen necessary for respiration from the air held in solution 

 by the water, and their food consists entirely of the minute living 

 creatures that always abound in natural waters. 



Again, we shall find that some of our live bivalves have 

 protruded a thick, conical, fleshy mass the foot, from the opposite 

 end of the body. This organ is the means of locomotion in the 

 case of the burrowing and other free-moving bivalves, but is 

 developed to a less extent in those species that lead a sedentary 

 life. Thus, the common Edible Mussel secretes a tuft of strong 

 silky fibres (byssus) by 

 means of which it fixes 

 itself to a rock or other 

 body, and therefore does not 

 need the assistance of a 

 muscular foot ; and an ex- 

 amination of its body will 

 show that the foot is very 

 small in proportion to the 

 size of the animal, as com- 

 pared with that of the 



wandering and burrowing Fio. 130.-Jf^t rfufo, 

 species. The same is true 



of the oyster, which lies fixed on its side, the lower valve being 

 attached to the surface on which it rests. 



We have made use of the terms dorsal and ventral in speaking 

 of the shell of a bivalve, and it is important that these and a few 

 other similar terms be well understood by those who are about to 

 read the descriptions of the animals, or who may desire to describe 

 them themselves. To do this, take a bivalve in your hand, and 

 hold it before you in such a position that the hinge is uppermost, 

 and the siphons turned towards you. The foot of the animal is 

 now pointing in the direction you are looking, and the mouth, 

 situated at the base of the foot, is also directed the same way. You 

 have now placed the shell, and, of coarse, also the animal, in such 

 a position that its dorsal side is uppermost, the ventral side below, 

 the anterior end turned from you, the posterior (often narrower) 

 end towards you, the right valve on your right, and the left valve 



