328 CHARACTERS OF INVERTEBRATE ANIMALS 



no trace of a gill can be seen, but the thin Tnantle which forms 

 its roof is raised up into a net- work of ridges traversed by blood- 

 vessels and acting as a lung. The intestine runs down the right 

 side of the mantle-cavity to its termination close by the lung- 

 opening, and by its side runs the slender tube which carries off 

 the waste matter from the single kidney, abutting against which is 

 the pericardium containing a two-chambered heart. 



The nerve-ring encircles the beginning of the gullet just 

 behind the pharynx, and the nerve-loop is exceedingly short, and 

 fused with it in such a way as to be exceedingly difficult to re- 

 cognize. About 6000 species are included in the Pulmonata, and 

 of these some 3500 belong to the same genus {Helix) as the 

 Garden Snail. A much larger species than this is the Roman 

 Snail {Helix pomatia), common abroad, and on the chalk downs 

 of Kent and Surrey. It is said to have been introduced in 

 Roman times for culinary purposes. 



Land-Slugs may be regarded as derived from snail-like 

 forms which have been more or less flattened out, and in which 

 the shell is reduced or, it may be, absent altogether. Two 

 common British species may be mentioned as examples, the 

 small grey Field -Slug (Limax agrestis) with a reduced internal 

 shell, and the much larger Black Slug {Arion ater) in which the 

 shell is entirely absent. 



The Pulmonate forms so far mentioned all agree in the pos- 

 session of four tentacles, with eyes situated on the tips of the 

 larger posterior ones. But there are still other forms in which 

 only one pair of tentacles is present, at the bases of which the 

 eyes are placed. Among these may be mentioned the Pond- 

 Snail {Limncsa stagnalis) with a thin, pointed shell, and the 

 Trumpet- Shell {Planorbis corneus), also an inhabitant of fresh 

 water, and possessing a flat spiral shell. 



Class 3.— BIVALVE MOLLUSCS (Lamellibranchia) 



The shell of a Gastropod, when it possesses one, always 

 consists of one piece or valve, i.e. is univalve; but in the class 

 now to be considered there is a bivalve shell consisting of a right 

 and a left valve. The Fresh-water Mussels, abundant in many 

 of our streams, canals, and ponds, furnish a convenient type. 

 They belong to two genera, Anodon and Unio, which agree in 



