CLASSIFICATION OF GASTEROPODA 



present, is never spiral, but consists of eight overlapping plates, 

 kept together by an elliptical girdle. The Amphineura are 

 divided into (a) Polyflacoi^liora^ or Chitons, and (5) Aplaeophora 

 (^Chaetoderma and Neomenid). 



(2) The Prosohrayicliiata'^ are so named from the fact that 

 the breathing organ (branchia or ctenidium-^) is as a rule situated 

 in front of the heart, the auricle at the same time being in front 

 of the ventricle. They are asymmetrical, almost always fur- 

 nished with a shell, which is at some time spiral, and with an 

 operculum. The sexes are separate. They are either marine 

 animals, or can be shown to be more or less directly derived 

 from genera which are marine. They are divided into (a) 

 Diotocardia^ (^Haliotis^ Fissurella, Trochus, Merita, Patella'), 

 which have, or whose immediate ancestors are believed to have 

 had, two auricles to the heart, two sets of breathing organs, two 

 kidneys, but no proboscis, 



penis, or siphon, and (5) 

 Monotocardia,^ in which 

 the heart has only one 

 auricle, the true breathing 

 organ is single, and there 

 is a single kidney. To this 

 division belong the great 

 majority of marine uni- 

 valve Mollusca, e.g, Cy- 

 praea, Buccinum, Murex, 

 Littorina, lanthina, all the 

 land and fresh-water oper- 

 culates (^Cyclostoma, Me- 



lania, Paludina, etc.), as well as the Heteropoda, which are a 

 group of Prosobranchiata which have betaken themselves to 

 a pelagic life. 



(3) In the OpistJiohranchiata ^ the breathing organs (when 

 present) are behind the heart, and the auricle of the heart is 

 consequently behind the ventricle. They are asymmetrical 

 marine animals ; usually, but by no means always, without a 



Fig. 4. — Example of a Heteropod, Cariaaria 

 mediterranea Lam., Naples: a, anus; hr, 

 branchia;/, foot; i, intestine; m, mouth; 

 /), penis; s, sucker; sh, shell; t, tentacles. 

 X5. The animal swims foot uppermost. 



1 TToXiys, many ; ttXci^, plate. 



2 irpocrw, in front. Often alluded to in the sequel as 'operculate Gasteropoda.' 



3 KTevidiov, a little comb. ^ 5uw, two ; yiovos, single ; wra, auricles ; Kapdia, heart. 

 ^ 6irL<T6ev, behind. 



