IX MOLLUSC A 109 



the base of these, on the inner front side, is a pair of simple 



speck-like eyes, very slightly raised on little fleshy tubercles.^ 



The head and foot, after fusion, are continued 



Humn* as the nech, which passes up under the shell and 

 is lost to sight. The body continues right up to 

 the very top of the spire of the shell, following its coils ; this 

 upper part of the body is known as the visceral hump, for in it 

 are contained the viscera — the intestine, kidney, heart, liver, 

 and the reproductive organ, which produces both egg cells and 

 sperms — for this snail, like the earthworm, is hermaphrodite. 

 (Most snails, but not all, agree in this characteristic.) 



Mantle The visceral hump is covered with a thin in- 



and Shell tegument or skin. In the body whorl, a fold of 

 Formation, ^jjjg integument grows down round the body, 

 loosely covering it. This fold, which is known as the mantle, 

 adheres closely to the inner surface of the shell, and ter- 

 minates at its open margin in a thickened rim known as the 

 collar. The " collar " is glandular, and from its secretions 

 are formed the outer horny and the middle calcareous layers 

 of the shell, whilst the inner nacreous or pearly layer is 

 formed by secretion from any part of the surface of the 

 mantle. If therefore the shell is damaged at its free edge, it 

 can be completely repaired, all three layers being renewed, 

 but if part of the shell is removed higher up, the hole can be 

 filled by the formation of fresh nacreous matter alone. 



Mantle There is a space between the mantle and the 



Cavity and dorsal wall of the body, known as the mantle cavity. 

 Respiration. ^^^^ jjjg communication of this space with the 

 exterior is narrowed down to a small, round, tubular aperture 

 (see Fig. 6 1, r), by the partial fusion of the collar with the body. 

 The aperture lies on the right side of the body in Limnaea. 

 This mantle cavity is filled with air, and functions as a 

 respiratory organ or lung, the mantle which forms the roof 

 and sides of the cavity being richly supplied with blood- 

 vessels, to which is brought all the impure venous blood of 

 the body. Here it comes into close contact with the air 

 in the mantle chamber, and the blood is aerated and then 

 carried by a special vessel to the heart, whence it is again 

 distributed to all parts of the body.^ 



' For the structure of this eye see p. 128. 

 ^ For similar lung chamber in the garden snail see p. 129, Fig. 84. 



