PATELLA. 257 



over so that it points forwards and is usually placed in front 

 of the middle of the shell. In adult Cyclobranchs growth 

 of the front edge of the shell often makes these features less 

 distinctly marked. 



(13) The long raking radula with a limited number of 

 specialised clawed teeth. The median tooth is not specially 

 differentiated (except in some Lepetidfe), and is typically 

 reduced and often absent. 



(14) The disposition of the parts of the gut shown in 

 Fig. 10a, except as regards details mentioned in (1) above. 

 This is not quite constant throughout the group, but is 

 found in several types and the disposition of the stomach- 

 coil is quite characteristic. 



(15) The position of the visceral loop of the nervous 

 system well to the right of the median line. 



(16) The practically triangular pericardium completely 

 filled by the heart and situated at the left anterior corner of 

 the dorsal surface of the visceral hump. 



(17) The superficial extension of the right kidney (almost 

 characteristic) and the retention of a certain amount of 

 functional excretory tissue by the tiny left kidney. 



The following features show that Patella and the Doco- 

 glossa rank among the lower Gastropods : — 



(19) The presence of a horse-shoe muscle derived, as the 

 development of Acmcea shows, from the backward extension 

 of a pair of laterally placed muscles. 



(20) The retention of external symmetry throughout the 

 development, as far as is known. 



(21) The feeble concentration of ganglia in the nervous 

 system and the anastomosing commissures between the 

 pedal cords. 



(22) The presence of a complete labial commissure 

 and the form both of this and of the buccal nervous 

 system. 



w 



