vm 



PHYLUM MOLLUSCOIDA 



357 



The body-wall consists externally of an epidermis formed of a 

 single layer of cells, then of a layer of connective tissue, of a 

 cartilaginous consistency in many parts, and finally of a ciliated 

 ccelomic epithelium lining the 

 body-cavity. On the outer 

 surfaces of the mantle-lobes, 

 where they are in contact 

 with the shell, the epidermis 

 is replaced by a thin mem- 

 brane showing no cell- 

 structure. 



The muscular system 

 (Fig. 298) is well developed. 

 Two large adductor muscles 

 (ad. m) arise on each side 

 from the dorsal valve, and 

 passing downwards, unite 

 with one another so as to 

 have a single insertion on the 

 ventral valve : their action is 

 to approximate the valves 

 and so to close the shell. A large and a small pair of divaricators 

 (d.m, d.m') arise from the ventral valves, and are inserted 

 into the cardinal process, which they depress : as this process 

 is situated posteriorly to the hinge-line, its depression raises 



d.v 



FIG. 297. Magellania'flayescens, the vcntra 

 valve removed, c. p. cardinal process ; Iph. arm 

 of lophophore ; Iph 1 . its coiled process, with 

 the tentacles removed on the right side ; mth. 

 mouth. (After Davidson.) 



m,' 



s.1! 



d.m, 



FIG. 298. Muscular system of MageUania. ad. m. adductors ; b. beak ; d. aj. m. dorsal 

 adjusters ; d. m., d. m'. divaricators ; d. v. dorsal valve ; int. intestine ; mth. mouth ; p/1. 

 peduncle ; pd. sh. sheath of peduncle ; p. m. protractor ; s. 1. shelly loop ; v. aj. m. ventral 

 adjusters ; v. v. ventral valve. (After Hancock.) 



the rest of the dorsal valve and so opens the shell. Two pairs of 

 muscles arising, one from the ventral, the other from the dorsal 

 valve, and inserted into the peduncle, are called adjusters (aj. m) : 



