GASTROPODA 



529 



The foot forms a broad sole, except in the Heteropoda: in a few 

 forms it can envelop the shell (Natica). The head, which often projects 

 snout-like forwards, bears a single pair of non-retractile tentacles and a 

 pair of eyes, which are either at the base of or on the tentacles or at the 

 tips of stalks of their own. The higher prosobranchs are distinguished by 

 the possession of a proboscis, which can often be thrust out far in advance 

 of the animal, and also a siphon, which is formed by a prolongation of the 

 anterior margin of the mantle, and in many snails lies in a characteristic 

 prolongation of the shell called the anterior or siphonal canal (Fig. 872, 1). 

 Paired jaws are present in the non-carnivorous forms, the carnivorous 

 ones being destitute of jaws. The radula varies much among the different 

 groups and is wanting in the Gymnoglossa. 



The nervous system is strep toneurous, the pleurovisceral connectives 

 crossing each other. The sexes are separate, the male being usually dis- 

 tinguished by a slenderer shell. The eggs are laid usually in capsules 

 (Fig. 879), which vary much among the various forms, and are often 

 attached to rocks or other objects. Littorina deposits its eggs in 'a jelly. 

 The fresh-water Viviparidae bear their young alive. 



The Prosobranchiata include about 27,500 living species, mostly marine, 

 which are grouped under two suborders. 



Key to the suborders of Prosobranchiata: 

 a t Gill bipectinate (Fig. 821, B) or absent, shell without siphonal canal 



(Fig. 872, 1) 1. SCUTIBBANCHIATA 



o a Gill monopectinate (Fig. 821, C) 2. PECTINIBBANCHIATA 



SUBORDER 1. SCUTIBRANCHIATA. (ASPIDOBRANCHIATA; DIOTOCARDIA.) 



Snails with a primitive 

 structure, a pair of ctenidia 

 being present in the Zygo- 

 branchiata, and in all others 

 the bipectinate ctenidia, 

 where present, projecting 

 freely anteriorly (Fig. 

 821, B) ; 2 auricles and 2 kid- 

 neys generally present, the 

 gonad opening to the outside 

 through the right kidney (except in the Neritidae) ; proboscis, siphonal 

 canal, and penis absent: 17 families, grouped in 2 divisions, inhabiting 

 the sea, fresh water, and the land. 



Key to the divisions of Scutibranchiata: 



di Shell conical and not spiral, and without a slit or hole 1. DOCOGLOSSA 



a a Shell mostly spiral, but when not so with slits or holes, . . .2. RHIPIDOGLOSSA. 



Fig. 821 Prosobranch gills. A, paired; B, 

 bipectinate, and C, monopectinate gills (Lang). 

 1, branchial vein; 2, auricle; 3, ventricle; 4, 

 aorta. 



