802 Mollusca. 



Order 1. Prosobranchiata. 



To this diyisioDj tlie essential characters of which, are set forth in 

 the preceding table, belong the majority of the shell-bearing 

 marine Gastropoda, some of the fresh-water forms and some 

 of the air-breathing terrestrial forms. Practically all the Proso- 

 branchiata possess a shell, usually a well-developed spiral, with or 

 without a notch or canal ; occasionally the shell is cap-like. An 

 operculum is usually present. There is generally a single gill within 

 the mantle-cavity. Some are herbivorous, others feed upon living or 

 dead animals.* 



1. Marine forms. This division is represented by numerous forms ; in 

 the tropics, especially, there are many large and beautiful species ; in colder seas 

 they are also numerous, but mostly small and inconspicuous. Among those 

 very common on British coasts may be mentioned: the Periwinkles 

 (Littorina), small, thick-shelled Gastropods, without the syphonal notch, which 

 may be found in great numbers on rocks close to the shore: the "Whelk 

 (Buccinum undatum), a large form, with a short syphonal canal, living in some- 

 what deeper water, and much used for bait : the Pelican's Foot (Aphorrhais 

 pes pelicani), the mouth of whose shell is prolonged into several claw-like 

 processes: Limpets {Patella), fontns with basin-like shells, which i-emain for 

 a long time on the same spot. 



2. Fresh-water forms. The following forms occui- in Britain and 

 elsewhere: the River Snail (Paludina vivipard), tolerably large (as much 

 as 4 c/m. high), with conical shell (Fig. 242), viviparous ; the young when bom 

 are similar to the adults, and nearly as large as peas ; the eggs are each"- enclosed 

 in a capsule containing a rich supply of albumen, and remain in the much-widened 

 oviduct of the female ; metamorphosis occurs vsdthin the egg-capsule. The 

 aUied genus Bithynia is oviparous, and comprises smaller forms. Another 

 species, Neritinia, belongs to a family also met vjith in the sea ; the shell is 

 hemispherical, the inner edge of its opening flattened. 



3. Cyclostoma elegans is a terrestrial form occurring in Great Britain ; 

 it breathes by means of a lung, but may easily be distinguished from the 

 Pulmonata by the presence of an operculum. 



The Heteropoda constitute a peculiar pelagic group of the Prosobranchiata. 

 They are almost transparent and large-eyed, with a large compressed foot, 

 by means of which they move through the water ; the foot is a pei"pendicular 

 muscular plate, with a sharp ventral edge, and has only retained the ordinary 

 gastropod condition at one spot, in the form of a sucker situated at its edge 

 (the sucker may, however, be absent). In some cases the visceral hump is very 

 well developed, and enclosed in a compressed discoid spiral shell ; the foot carries 

 an operculum, and can be withdrawn into the shell. In others (Fig. 248) the 

 visceral sac is small, and provided only with a cap-shaped shell, whUst the lower 

 portion is relatively huge, has no operculum, and naturally cannot be with- 

 drawn into the shell. Lastly, there are some in which the visceral mass is still 



* In certain Prosobranchiata (Natica) there is, on the lower side of the 

 proboscis, a sucker-like area, the epithelium of which secretes an acid. When the 

 animal lays this against the shell of another MoUusc it forms a hole in the latter, 

 through which the proboscis can enter to devour the soft portions of the prey. 



