370 



PLANORBIS ; LIMN.EA ; AKCYLU8. 



about eight inches in length ; the eggs of these gigantic Snails 

 are more than an inch long, and enclosed in a brittle, calcareous 

 shell. In certain species of the latter genus, the direction of the 

 coils of the shell is opposite to what it is in other spiral shells ; 

 such are said to be reversed. A European species is one of those 

 most remarkable for the decollation of its shell. 



983. The aquatic Pulmonifera have only two tentacula ; and 

 the eyes are placed upon the head, at the base of these, instead of 

 being supported at the extremities of tentacula. They are, of 

 course, under the necessity of coming frequently to the surface to 

 breathe ; and usually live in water of inconsiderable depth, such 

 as ponds and small streams. They feed on the leaves of aquatic 

 plants, especially when in a state of incipient decay. The 



LiMNvEiD^E, including 

 the well-known Pond- 

 snails, have a thin shell 

 with a simple aperture. 

 : They are exceedingly 

 abundant in ponds in 

 this country, living for 

 the most part amongst 

 the weeds ; but they are 

 also capable of floating 

 at the surface of the wa- 

 ter with the shell down- 

 wards, and when in this position can even glide slowly along by ths 

 action of the foot. The P/anorbis, the shell of which is quite flat 

 having all its coils upon the same level, is a very common genus 

 in this country ; as is also the Limncea, which feeds upon seeds, 

 as well as the softer parts of plants, and the stomach of which 

 has a very muscular gizzard. The curious genus Ancylus has a 

 limpet-like shell ; the species live in streams, and are known as 

 River-limpets. The AURICULID^E closely resemble the Limnaei- 

 da3, but have a stouter shell, with the margins of the aperture 

 toothed. The species are generally found in the brackish water 

 of salt-marshes and tidal streams ; some of them even frequent 

 the sea-shore. A few species, are terrestrial in their habits. The 

 CYCLOSTOMID^E are distinguished from the preceding Pulmoni- 



FIG. 664. LIMNJSA STAGXAI.IS. 



