684 



MOLLUSCA. 



The body in the Lamellibranchs is bilaterally symmetrical, and is 

 enclosed in a largely developed "mantle" or " pallium," which is 



divided into two lateral halves, the 

 right and left " lobes " of the mantle. 

 The two lobes of the mantle are 

 united along the dorsal side of the 

 body, and are prolonged laterally as 

 two great flaps, which conceal the 

 body, and enclose inferiorly a cham- 

 ber known as the "mantle-cavity." 

 The lower or ventral edges of the 

 mantle-lobes are normally free ; but 

 in many Bivalves they become more 

 or less fused with one another (fig. 

 552), so that the animal is enclosed 

 in a complete sac, in which certain 

 openings are left anteriorly and pos- 

 teriorly. Two of these apertures are 

 at the posterior end of the animal, 

 and serve to permit the ingress into 

 the mantle-cavity of the water required 

 for respiration and for the purpose of 

 obtaining food, and the egress of the 

 same. The lower or ventral aperture 

 is inhalant in function ; the upper or 

 dorsal aperture is exhalant ; and the 

 anus is always placed in the vicinity 

 of the latter, so that excrementitious 

 matters are carried away in the out- 

 going currents of water. In many cases these two openings into 

 the mantle-sac are drawn out into longer or shorter muscular tubes, 

 which are known as the "siphons." In those Bivalves which have 

 the ventral margins of the mantle-lobes free, the in-going and out- 

 going currents of water still enter and leave the mantle-cavity by 

 openings at its posterior end. 



In those Bivalves which have the mantle-lobes fused along their 

 ventral edges, a third aperture is necessary in order to allow of the 

 protrusion of the " foot." This aperture is always placed ventrally 

 and towards the anterior end of the animal. In the forms with 

 free mantle-lobes no special opening is needed for the protrusion of 

 this organ. 



The "foot" of the Bivalves is not so extensively developed as in 

 the Gastropods. Usually it forms a hatchet-shaped muscular organ, 

 which may be used in locomotion, but is hardly ever adapted for 

 crawling. In other cases it is cylindrical in shape, and in the 



Fig. 552.— Diagrammatic vertical and 

 transverse section of Mya arcnaria. b, 

 Back, or "dorsal margin" of the shell ; 

 .y s, The two valves of the shell, right 

 and left ; 111 m, The two halves, or 

 "lobes," of the mantle, producing the 

 shell; gg, The gills, two pairs on each 

 side ; h, The heart; i, Intestine; ^ The 

 foot. 



