vin PHYLUM MOLLUSCOIDA 333 



freely into the cavity enclosed between the two valves, and has the 

 form of a simple loop bent upon itself. The inside of the dorsal 

 valve also has muscular impressions. 



Externally both valves present a series of concentric markings 

 parallel with the edge or gape : these are lines of growth, the 

 shell being built up by new layers being deposited within those 

 previously formed, and projecting beyond them so as to form a 

 series of outcrops. 



Microscopically the shell consists of prismatic rods or spicules 

 of carbonate of lime, placed obliquely to the surface, and separated 

 from one another by a thin layer of membrane. It is also tra- 

 versed, perpendicularly to the surface, by delicate tubules which 

 begin on the inner surface in microscopic apertures, and extend 

 to within a short distance of the outer surface. 



The actual body of the animal (Fig. 266, B) lies at the posterior 

 end of the shell, occupying not more than a third of the space 

 enclosed between the two valves : it is consequently more or less 

 wedge-shaped in form, and presents dorsal and ventral surfaces in 

 contact with the two valves, and an -anterior surface looking 

 towards the gape. The dorsal is of greater extent than the 

 ventral surface, so that the anterior surface is placed obliquely. 



The dorsal and ventral regions are continued each into a flat 

 reduplication of the body-wall, closely applied to the correspond- 

 ing valve and containing a prolongation of the coelome. The two 

 flaps thus formed are the dorsal (d. m) and ventral (v.m~) mantle- 

 lobes. They are fringed with minute setae (s) lodged in muscular 

 sacs, like those of Chsetopods (vide infra), and give off from their 

 outer surfaces hollow processes which extend into the tubules of 

 the shell mentioned above. 



The large wedge-shaped space or mantle-cavity, bounded by the 

 mantle-lobes above and below, and behind by the anterior surface 

 O^^th-e bo^cta is occupied by a huge and complex lophophore (Figs. 

 2^6 and 2&7, Ipli), which springs from the anterior surface of the 

 body, and, like that of the fresh-water Polyzoa and of Phoronis, 

 has the general foim of a horse-shoe. It is, however, peculiarly 

 modified : the two limbs of the horse-shoe curve towards one 

 another so as to adapt themselves to the mantle-cavity, and the 

 middle of the concave edge, which is dorsal in position, is pro- 

 duced into a spirally coiled offshoot (Iph'} which lies between the 

 two arms, and is coiled towards the dorsal side. The lophophore 

 is hollow, containing a spacious cavity or sinus : its two main arms 

 also receive prolongations of the coelome into which the digestive 

 glands project: it is fringed throughout its whole extent with 

 long ciliated tentacles which form the outer boundary of a ciliated 

 food-groove, bounded on the inner side by a wavy ridge or lip 

 (//>. ///). By the action of the cilia microscopic particles are swept 

 along the food-groove to the mouth. 



