MOLLUSC A 



129 



of the tentacle, can be seen in Fig. 82, which represents 

 a section of the head taken through the long tentacles, and 

 which also shows the short tentacles in front. 



The mouth is provided with a single hard upper jaw 

 against which the rasp works. It shows as a black curved 

 bar when the mouth opens (Fig. 

 83). The rasp has 107 teeth in 

 each row, and 145 rows, so that 

 with such a formidable weapon to 

 use the damage done by this snail 

 is not surprising. 



The "lung" cavity lies in the 

 body whorl, and opens to the ex- 

 terior on the right side. The 

 aperture is easily seen, and stays open for a consider- 

 able time. The method of breathing is similar to that 

 described on p. 109 in the account of the pond snail, 

 Limnaea. It is illustrated by Fig. 84, in which part of 



FIG. 83. Helix 

 much enlarged. 



FIG. 84. Helix dissected from the right side. 



r, Respiratory cavity ; m, vascular mantle ; cs, cut shell ; h, heart ; bv, blood-vessel 

 carrying blood to the heart ; fc, kidney ; gg, genital groove ; g, genital. 



the right side of the body is supposed to be cut away, thus 

 exposing the respiratory cavity and the structures in relation 

 with it. 



The blood of Helix is colourless when impure, but blue 

 when oxidised, for the oxygen carrier, which is dissolved in 

 the blood, is not haemoglobin but haemocyanin, a copper 

 compound. This is characteristic of most snails, though in a 

 few, e.g. Planorbis, haemoglobin is found and the blood 

 is red. 



VOL. i K 



