306 



Mollusca. 



1. Tie terrestrial Pulmonata (Stylommatophora) are chairacterised 

 by the position of tte eyes, which are at the tips of eye-stalks, exactly similar 

 to the tentacles, and like these, capable of invagination into the head. To this 

 group belong both shell-bearing and naked forms. Among the former may be 

 mentioned the genus ifeZiic, to which the small Garden Snail (H. hortensis), 

 and the large Edible Snail {H. pomatia) belong; among the latter are the 

 Great Black-slug {Arion ater), and the smaller, destructive, grey. Common Slug 

 (lAinax agrestis). 



2. The freshwater Pulmonata (Basommatophora) have sessile eyes, 

 situated at the base of the tentacles, which cannot be invaginated. To this 

 group belong the numerous species of the Pond Snail (Limnseus), with pointed 

 shell, and of Planorbis, with discoid shell ; both genera are common everywhere 

 in fresh water, and are represented by large and small species : some of the 

 Limnaeida) lead an amphibious life, being met with on land as well as in water. 



Class 3. Acephala (Zamel/idranch-) 



The body of the LamellibrancliSj unlike that of the Gastropoda, 

 is usually bilaterally symmetrical, except as regards the 

 coils of the alimentary canal. The anus is posterior, the genital 

 and nephridial apertures, etc., are symmetrically disposed. The 

 following is a general account of the structure of a Lamellibranch : 

 the body proper is rather small in comparison with the whole 

 size of the animal ; the mantle-fold is developed on either side 

 in the form of a large lamina, dependent like a curtain. On each 

 side, also, just below the point of origin of the mantle, two gill 

 lamellae arise and hang down within it. The foot is ventral, 



and is usually keel-shaped. There 

 are four large labial palps in 

 front of the mouth. A specialised 

 head is absent. The whole animal 

 is generally enclosed in a symmetrical, 

 laterally compressed, bivalve shell. 



The foot is, as a rule, not very 

 well marked off from the trunk ; most 

 often it is only a compressed longi- 

 tudinal keel on the ventral surface. 

 In some forms it is longer and more 

 projecting (sometimes geniculate) ; in 

 a few cases it is linguiform (Mussels) ; 

 in others there is a true pedal disc. 

 It is the most important locomotor 

 organ of the Lamellibranchs, and can 

 be protruded from the shell by being 

 distended with blood, whilst if the 

 animal is disturbed, it is withdrawn 

 by means of muscles arising from the inner surface of the shell, 



Fig. 252. A transverse sec- 

 tion through a Lamelli- 

 b ran oh (diagram). b ligament, / 

 foot, g gUl, h heart, hs pericardium, 

 le mantle, n kidney, s shell, t gut. — 

 Orig. 



