lO 



CLASSIFICATION OF GASTEROPODA 



CHAP. 



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

 sexes are united in the same individual. The Opistliohrayiehiata 

 fall into two divisions : (a) Tectibranchiata, 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, A Tectibran- 

 chiate Opisthobrauch, 

 Umbrella mediterra- 

 nea Lam., Naples: a, 

 anus ; b)\ branchia ; /, 

 foot; m, mouth; 7'h, 

 rhinopliores ; sh, shell. 



B, A Pteropod, Hya- 

 laea tridentata Forsk., 

 Naples : sh, shell ; /, I, 

 swimming lobes of 

 foot. 



C, A Nudibranchi- 

 ate Opisthobrauch, Ae- 

 oils peregriiiay'i^^aples: 

 f, foot ; c, cerata. 



Aplysia., Umbrella, and the whole group of Pterojyoda ; (5) 

 Nudibranchiata, 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 Pidmonata^ are asymmetrical air-breathing non- 



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

 pere^r?'^ Miill. : e,e,eyes; ^,?, tentacles. B, Pulmonata Stylommatophora, ^e//x 

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

 of the pulmonary orifice in Limnaea wiU 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 Puhno, a lung. 



