MOLLUSC A 



113 



Although Limnaea, like all air-breathing snails, 

 Tfon."° ^^ hermaphrodite, cross fertilisation takes place, 



the sperm cells being passed from one snail to 

 another through a special 

 tubular structure known 

 as the "penis." 



The eggs are laid at in- 

 tervals during the summer ; 

 they are deposited about 

 thirty at a time, embedded 

 in a curved mass of jelly, 

 which is nearly an inch 

 long, and is usually de- 

 posited on some water- 

 weed to which it adheres. 

 The young snails hatch in 

 about a month. They do 

 not at once need to rise to 

 the surface for a supply of 

 air, for they are hatched 

 with the lung-cavity full 

 of water, and probably 

 they are capable of respir- 

 ation through the skin, 

 using the air dissolved in the water. 



The growth of the young snail is fairly rapid 



at first ; in three months the shell may be nearly 

 an inch long, but the full size is not attained for two 

 years. The rate of growth depends on the temperature 

 and the volume of the water in which the snail is living, 

 as well as on a plentiful supply of food ; the optimum 

 temperature for growth is said to be about 25° C. ; below 

 12° 0. the snail may exist quite healthily, but will not grow. 

 The variation in size according to the volume of water 

 in which the snail is reared is very marked — even after 

 a week a difference can be detected between the size 

 of snails kept in a small vessel and others from the 

 same batch of eggs kept in a much larger tank ; the less 

 the amount of water the smaller the shell, other con- 

 ditions being similar and favourable. Limnaea may live 

 five years. 



VOL. I I 



Fig. 64. — Limnaea stagnalis. 



Two masses of eggs (e and e') attached to a 

 branch of water-crowfoot. 



Growth. 



