TYPE MOLLUSC A. 



277 



of the body there is a special thickening of the muscle-tissue 

 to form a "foot" (Fig. 122, p), which assumes a great variety 

 of forms, and special muscles are developed for its protrac- 



!/pe zl k 7 



FIG. 122. DIAGRAMS SHOWING THE ARRANGEMENT OF THE ORGANS IN AN 



IDEAL MoLLUSK (after LANKESTEK). 



a tentacle. i = ctenidium. 



b = head. k = reproductive pore. 



c = margin of mantle I = nephridial pore. 



d = margin of shell. in = anus. 



e = edge of body. n and p foot. 



/= edge of shell depression. r = coelom. 



g = shell. s = pericardium. 

 gc = cerebral ganglion. t = testis. 



gpe = pedal ganglion. u = nephridium. 



gpl = pleural ganglion. v = ventricle of heart. 



h = osphradinm. zl = liver. 



tion or retraction when this is necessary, as well as for the 

 closure of the shell in those forms (Pelecypoda) in which it is 

 a bilateral structure. 



The ccelom is in some forms a relatively spacious cavity, 

 traversed, however, even in these cases by thin bands of con- 



