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) PolyiJlacophora^ or Chitons, and {h) Aplacoijhora 

 (Chaetoderma and NeoTncnia). 



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

 the breathing organ (branchia or ctenidium •') is as a rule situated 

 i7i front of the heart, the auricle at the same time being in front 

 of the ventricle. They are asymmetrical, almost always furnished 

 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) Diotocardicf^ 

 (Raliotis, Fissurella, Trochus, Nerita, 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 (?>) Monoto- 

 cardio ,^ 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 univalve Mol- 

 lusca, e.g. Cypraea, Bvxci- 

 nurn, Murex, Littorina, 

 lantliina, all the land and 

 fresh - water operculates 

 {Ci/clostoma, Melania, Fa- 

 iled ina, etc.), as well as the Ifcterojwda, which are a group of 

 Prosobranchiata which have betaken themselves to a pelagic life. 



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

 present) are heJtind 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 

 sliell, scarcely ever with an operculum in the adult state. The 



Fig. 4. — Example of ; 

 mediterranea Lam., 

 branchia ; /, foot ; 

 J), penis ; s, sucker 



Heterojiod, Carinaria 



Naples : a, amis ; br, 



intestine ; m, mouth ; 



sh, shell ; t, tentacles. 



The auimal swims foot uppei'most. 



^ TToXi's, many ; 7r\d^, plate. 



- irpdaw, in front. Often alluded to in the sequel as 'opereulate Gasteropoda.' 



^ KTividiov, a little comb. ■* dvu, two ; fj.6vos, single ; Sira, auricles ; Kapdia, heart. 



■' o-maOiv, behind. 



