128 



INTRODUCTION TO ZOOLOGY 



CHAP. 



The colour of the body is a dark brown or 



suckled with 

 the milky-white 

 spots which give to it its specific 

 name, " aspersa," signifying 

 " sprinkled." The foot has a 

 yellow band round it. 



The form of the body of this 

 snail is very similar to that of 

 Limnaea, although in Helix 

 there are four tentacles on the 

 head instead of two, and at the 

 tip of each of the longer upper 

 pair is a fairly complex eye, 

 which can be readily seen as a 

 little black dot. 



This eye contains a solid lens, 

 behind this a pigmented layer, and then a layer of sensory 



cells in connection with the 

 optic nerve (see Fig. 81). 

 In spite, however, of the 

 considerable complexity of 



FIG. 81. Longitudinal Section 

 through the tip of one of the 

 long tentacles of Helix to show 

 the structure of the eye. 



e, Epidermis ; L, lens ; p, pigmented 

 layer ; r, retina ; op, optic nerve. 



t.2 



the 



organ, 



the 



sense 



of 



sight does not seem at all 

 keen. 



The tentacles are hollow, 

 and can be retracted by 

 means of a special muscle 

 which is visible through the 

 semi-transparent wall of the 

 tentacle, running inside it 

 from base to apex (see Fig. 

 82). 



When this muscle con- 

 tracts, the tip of the tentacle, 



FIG. 82. Longitudinal Section of the .,, :, L . , -, 



Head of a Snail. (Diagrammatic.) with the eye, is drawn down 

 (After Furneaux.) inside the tentacle, and so 



ft, Long tentacle extended ; 2 f long tentacle the CVC is protected. The 

 withdrawn ; $ t* short tentacles ; in, . . , V i 



muscles ; n, nerves. pOSltlOll OI the eye and OptlC 



nerves, and the muscles 

 which cause by their contraction the turning " outside in " 



