GASTEROPODA. 373 



foot usually contains a mucus gland, and tends to be divided 

 into three regions — the pro-, meso-, and meta-podium. There 

 is a single reproductive organ and genital duct. 



Order 1. Prosobranchiata. 



A shell is almost always present, and the foot frequently bears an 

 operculum. The pleuro-visceral commissure is twisted into the form of 

 the figure 8 (streptoneural). There is usually only one gill lying in front 

 of the heart. When one auricle is present it lies in front of the ventricle. 

 Sexes separate. 



A. Diotocardia. Primitive forms. The heart has usually two 



auricles, and there are two nephridia ; Zeugobranchs, with 

 two gills, e.g. Haliotis ; Azygobranchs, a single gill , 

 Tarda, Trochus, etc. ; Docoglossa, single gill, and single 

 auricle, left nephridium degenerate, no operculum, e.g. limpet 

 (Patella), without gill (Fig. 161) ; Acmiea, with single gill. 



B. Monotocardia. Heart with single auricle, one gill, one 



nephridium; operculum present. Periwinkle (Littorina), 

 buckie (Buccinum, Fig. 158), Dog -whelk (Purpura), 

 lanthina, 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. 



Order 2. Pulmonata. 



The visceral loop is short and untwisted (euthyneural), gills are 

 absent, and the mantle cavity functions as a lung ; all are hermaphrodite, 

 e.g. the snail (Helix) ; the grey slug (Limax) ; the black slug (Arion) ; 

 fresh-water snails, such as Linmieus, Planorbis, and Ancylus. 



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, Dolabella, Umbrella. The Tectibranchiata 

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

 flies, which have become much modified for pelagic life. 

 They have a secondarily acquired apparent symmetry, and 

 swim by two large lateral lobes of the foot (" parapodia"). 

 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 re- 

 lated to Bulla and its allies. 

 Examples. — Hyalea, Cymbulia. 



