THE MOLLUSCA 99 



than the shell itself, and is armed with as many as 1,920 tiny 

 hooked teeth arranged in 160 rows of twelve teeth each. The 

 Whelk has from 220 to 250 teeth, the common Periwinkle about 

 3,500, the Garden Snail (Helix aspersa) owns about 15,000 teeth, 

 and a large Mexican Snail (Helix Ghiesbreghti) is the proud possessor 

 of 40,000. But even this extraordinary number is beaten by the 

 curious Umbrella Shells, some species of which boast more teeth 

 than it is possible to count, though the number is roughly calcu- 

 lated to be somewhere about 750,000. 



This curious tongue is used by the Mollusc to rasp and tear 

 the food, which is first bitten off by the horny jaws, the morsels 

 of food as they pass along the tongue to the gullet being rasped 

 and torn by the sharp, prickly teeth as they are forced onwards 

 by the worm-like movements of the radula. From this constant 

 rasping action the front rows of teeth often become worn away, 

 and their place is taken by the next rows in order which are 

 successively pushed up from behind. 



The head of a Gastropod Mollusc bears from two to six tenta- 

 cles arranged in pairs, one of which usually carries a pair of eyes, 

 placed either at the tip, as in the land Snails, or at the base, as 

 in the majority of marine species. In some cases, however, the 

 eyes are not carried on the tentacles, but are placed farther back 

 at the sides of the animal's neck. In some species the eyes are 

 altogether absent, while in others Natica, Sigaretus, and Doris 

 although the eyes are developed, they are covered with a thick 

 skin, rendering the animal practically blind. The eye in the 

 Gastropoda varies considerably and is more or less well developed 

 in different species. Thus, in the Limpets (Patella) the eye is a 

 mere depression lined with pigmented and retinal cells ; in Trochus 

 the eye is more advanced, the depression is deeper and filled with 

 a bright crystalline mass ; while the great Wing Shells (Strombictce) 

 of the tropic seas possess well-developed eyes with a distinct 

 pupil and a double iris, which is often wonderfully coloured, and 

 said to equal the eyes of birds and reptiles in beauty and correct- 

 ness of outline. 



Far the greater number of Gastropods are protected by an 

 external univalve shell, a few species have an internal shell, while 

 in others the shell is rudimentary or altogether absent, or it may 

 be partly external and partly internal. The Chitons differ from 



