lO 



CLASSIFICATION OF GASTEROPODA 



CHAP. 



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

 sexes are united in the same individual. The Opisthohranchiata 

 fall into two divisions : (a) Tectihranchiata^ in which the breath- 

 ing organ is more or less covered by the mantle, and a shell 

 is usually present, which is sometimes rudimentary, e.g. Bulla, 



Fig. 5. — A, ATectibran- 

 chiate Opisthobranch, 

 Umbrella mediterra- 

 nea Lam., Naples: a, 

 anus ; br, branchia ; /, 

 foot; m, mouth; rh, 

 rhinophores; s/i, shell. 



B, A Pteropod, Hya- 

 laea tridentata Forsk., 

 Naples : sh, shell ; I, I, 

 swimming lobes of 

 foot. 



C, A Nudibranchi- 

 ate Opisthobranch, Ae- 

 olis peregrina,'Na}Aes: 

 /, foot ; c, cerata. 



Aplysia^ Umbrella^ and the whole group of Pteropoda ; (6) 

 Nudihranchiata^ or sea slugs, which have no shell and no true 

 ctenidia, but breathe either by the skin, or by ' cerata ' or papilli- 

 form organs prominently developed on the back: e.g. Doris, 

 Aeolis, Dendronotus. 



(4) The Fulmonata^ are asymmetrical air-breathing non- 



FiG. 6. — Examples of — A, Pulmonata Basommatophora, the common Limnaea 

 perer/ra Miill. : e, e, eyes ; t, t, tentacles. B, Pulmonata Stylommatophora, Helix 

 hortensis Miill.: e, e, eyes; t, t, tentacles; p. o, pulmonary orifice (the position 

 of the pulmonary orifice in Limnaea will be seen by reference to Fig. 101) . 



marine Mollusca, generally, but not always, furnished with a 

 shell. The sexes are always united in the same individual, and 

 the operculum is always wanting, except in Amphibola. They 



1 Pulmo, a lung. 



