474 



PHYLUM MOLLUSCA 



Order 2. Azygobranchiata 



The originally left gill and the originally left nephridium have been 

 lost. Heart with single auricle ; one gill, one nephridium ; operculum 



present. 



Periwinkle {Littorina), buckie (Buccinum, Fig. 264), dog-whelk 

 {Purpura), lanihina, and the majority of the marine Gasteropods 

 with coiled shells, together with some fresh-water forms. The 

 pelagic Heteropods are also included here -.—Atlanta, shell well 

 developed ; Carinaria, with small shell ; Pterotrachea, with no 

 shell. 



Branch B. Euthyneura 



The visceral loop does not share in the torsion of the visceral hump. 



Order 3. Opisthobranchiata 



The visceral loop is euthyneural, as in snails ; the single auricle 

 lies behind the ventricle ; the shell and mantle are often absent. 



A. Tectibranchiata. A shell is present, but may be rudimentary ; 



there is a well-developed mantle fold and a single gill, e.g. 

 Bulla, Aplysia, Dolahella, Umbrella. The Tectibranchiata 

 also include the Pteropoda, the winged snails or sea-butter- 

 fiies, which have become much modified for pelagic life. 

 They have a secondarily acquired bilateral symmetry, and 

 swim by two large lateral lobes of the foot. They often 

 swim actively in shoals, and occur in all seas. They afford 

 food for whales, etc., and the shells of some are abundant in 

 the ooze. They include — 



(a) Thecosomata, with mantle fold and shell, diet of 



minute animal or vegetable organisms, closely related 



to Bulla and its allies. 



Examples. — Hyalea, Cymhulia. 

 {b) Gymnosomata, without mantle fold or shell in the 



adult. Closely allied to Aplysia and its allies. 



Actively carnivorous, e.g. Clio, Pneumoderma. 



B. Nudibranchiata. Shell, mantle fold, and true gill are absent : 



various forms of " adaptive gills " may be present, or there 

 may be no special respiratory organs, e.g. sea-slugs, Doris, 

 Eolis, Dendronotus (Fig. 266). 



Order 4. Pulmonata 



The visceral loop is short and untwisted, gills are absent, and the 

 mantle cavity functions as a lung ; all are hermaphrodite, e.g. the snail 

 (Helix); the grey slug (L:w/a;i;) ; the black slug (.4 rjow) ; fresh- water 

 snails, such as Limnaa, Planorbis, and Ancylus. 



Mode of life. — From the number of diverse types which 

 the class includes, it is evident that few general statements 



