258 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



(23) The possession of a pair of osphradia placed right 

 and left in the nuchal cavity. 



(24) The possession of two kidneys right and left, the 

 left still possessing some excretory tissue. 



(25) The extrusion of the sexual products through the 

 right kidney and the ahsence of all accessory sexual organs. 



Of those characters which are peculiar to the Docoglossa 

 amongst Gastropods — 



No. (12) is an adaptation to the adhesive habit, while 

 (14), (15), and (17) are consequences of the consolidation of 

 the visceral hump involved in the development of (12). 



No. (16) is due to the disappearance of the right ctenidium 

 and the subsequent shifting of the heart to the lefl side so 

 that it might lie behind the remaining ctenidium (which has 

 also disappeared in Patella). 



No. (13) is a specialisation enabling the animal to gather 

 its food by scraping the rock or other surface over which 

 it creeps. 



Among the Cyclobranchs: — 



Nos. (1) and (5) are specialisations to overcome the 

 difficulty of digestion of the tough food. 



Nos. (2) and (7) are special consequences of the further 

 compression of the parts of the visceral hump among these 

 forms. 



No. (3) is aii undoubted characteristic, but is not easy to 

 understand. 



No. (4) is an adaptation to the habit of extruding the 

 tip of the odontophore for raking purposes and to the 

 consequent need of a flat dorsal surface, a broad front, and 

 facilities for adjustment. 



The reduction of the pericardium has led to the develop- 

 ment of No. ((>). this arrangement promoting the regular 

 and unimpeded circulation of the blood. 



No. (8) is the feature which gives the group of " Cyclo- 



