THE MOLLUSCA HI 



somewhere near the centre of the body, and on each side of the 

 mouth is a curious bunch of filaments, apparently used by the 

 molluscs for seizing their food ; they have, too, a large radula 

 to enable them to rasp it. 



In the order Pelecypoda, popularly called " bivalves," the 

 mollusc is enclosed in a shell which consists of two plates or valves. 

 The valves are connected by an elastic ligament and articulated 

 by interlocking teeth which form a perfect hinge. During the life 

 of the animal the valves are closed by one or two large muscles, 

 termed the adductor muscles. After death, when the muscles 

 cease to act, the valves gape open in consequence of the pull of 

 the elastic ligament. If we examine an empty bivalve shell we 

 shall see one or two large circular scars on the inner surface, which 

 are the points of attachment of the adductor muscles. We shall 

 also notice within the margin of the shell a single, unbroken line ; 

 this is called the pallial line, and marks the line of attach- 

 ment of the mantle of the mollusc to the interior of the shell. 



Bivalve Molluscs differ considerably from univalves ; they have 

 neither head, jaws, nor tongue (with a few exceptions). The body 

 of the animal lies within the valves of the shell between two folds, 

 or lobes, of the mantle which lines the interior of the shell. The 

 gills are leaf-like and disposed on each side of the body, and the 

 mouth is a simple opening at the anterior end, usually between 

 a pair of thin, fleshy lips or labial palps, through which the food 

 is conveyed. In some bivalves the edges of the mantle are quite 

 free, and when the valves are open the water readily flows within 

 the shell, carrying fresh supplies of oxygen to the gills, and food 

 to nourish the mollusc. The edges of the mantle may be either 

 simple, folded, or fringed with tentacles or cilia. In others the 

 mantle lobes are joined at one or more points, forming separate 

 openings for the in-flowing and out-flowing currents of water, 

 and for the passage of the foot, when this is present. The aper- 

 tures for the ingress and egress of the water may be simple open- 

 ings in the mantle, but in the majority of the Mollusca the orifices 

 are drawn out into long tubes or siphons through which the water 

 circulates. The siphons may be entirely free throughout their 

 length or partly fused together. 



Bivalves are sedentary creatures, and do not roam about in 

 search of food as do the Gastropods. Many species, after their 



