562 THE INVERTEBRATA 



a skin appears, and in this crystals of calcium salts are laid down 

 which aggregate to form a definite shell. The eggs are laid in July and 

 August in small holes in the earth and hatch after about twenty-five 

 days of development. 



In the autumn the snail loses its appetite and hides, often in 

 company with large numbers of its fellows, under leaves, making a 

 small hole in the ground with its foot and shell in which it lies with 

 the aperture upwards. The head and foot are withdrawn into the shell 



cent. 



Wflr.v>, 



lat. 



aamaa 



cent. 



Fig. 386. Radula of various types. A, Docoglossate (Patella). Stout teeth 

 used for rasping encrusting layer of algae off rocks: radula of relatively 

 enormous length; the teeth are quickly worn away. B, Rhipidoglossate 

 {Haliotis). Lateral and central teeth as in Patella, used in browsing on algae 

 growing on stones. The marginals, of which only about half are shown, are 

 probably used as a sieve to prevent fragments of food of too great size entering 

 the oesophagus. C, Rachiglossate (Buccinum). Teeth of carnivorous type, 

 with sharp cusps. D, Toxiglossate (Conus). Specialization of carnivorous 

 type, in which only two teeth (laterals) remain in each row, are hollow, and 

 are used as poisoned daggers, carrying the secretion of the salivary glands. 

 cent, central, lat. lateral, mg. marginal teeth. 



and the edges of the mantle approximate to form an almost complete 

 disc filling up the aperture, leaving only a small hole for breathing. 

 They secrete a membrane (epiphragma) mostly composed of Ca3(P04)2 . 

 Several such membranes may be found behind each other. In this 

 winter sleep the snail remains for about six months ; respiratory move- 

 ments are carried on slowly and the heart beats sink from about 

 10-13 to 4-6 per minute. The rate of heart beat is closely dependent 

 on the temperature, and at a temperature of 30° C. is from 50 to 60 

 beats per minute. 



