364 



ZOOLOGY 



SECT. 



.th. 



cartilaginous consistency in many parts, and finally of a ciliated 

 coelomic 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 membrane showing 

 no cell-structure. 



The muscular system 

 (Fig. 292) 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, dm') 

 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 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 



tyh. 



FIG. 291. IVIagellania flavescens, the ventral 

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

 of lophophore ; Iphi. its coiled process, with 

 the tentacles removed on the right side ; mth. 

 mouth. (After Davidson.) 



cL.m, 



FIG. 292. Muscular system of IVIagellania. ad. m. adductors ; b. beak ; d. aj. m. dorsal 

 adjusters; d. m., d. m'. divaricators; d. v. dorsal valve; int. intestine; ruth, mouth; pd. 

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

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



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

 peduncle being fixed, they serve to alter or adjust the position 

 of the animal as a whole by turning it in various directions. 



