206 ZOOLOGY 



Organs of Excretion. — Mollusks have well-developed Idd- 

 neys which are paired in most symmetrical species ; Init in 

 unsj-ninietrical species, like most gasteropods, only one is 

 present. In the snail the kidnej- lies close to the heart. It 

 opens on the one hand into the bod^'-cavity surrounding the 

 heart and, on the other, into the mantle-cavity near the anal 

 opening behind the right eye-stalk. 



Organs of Reproduction. — The sexes in moUusks are 

 usually separate, but in land-snails the eggs and sperm-cells 

 are jjroduced in different parts of one and the same gland. 

 The eggs on their ^\&y out recei\-e a large amount of albmuen 

 from a gland, and further down in the oviduct an egg membrane 

 is secreted. The eggs of land-snails are laid at various times 

 of the year in launches in moss or under boards and leaves, and 

 are clear, translucent little spheres al)0ut two millimetres in 

 diameter. 



Musculature. — The whole skin in the snail is highly muscu- 

 lar, but the muscles gain their greatest importance in the foot, 

 where they assist in locomotion. Important muscles run to 

 the head from the visceral region, and it is l^y them that the 

 head is retracted into the shell (Fig. 195, 5-7). 



Nervous System. — The nervous system of moUusks is very 

 different from that of annelids and Crustacea. There are a few 

 pairs of knots of ner'\'ous matter or gangUa. The two mem- 

 Ijers of a pair are united In' nerves, anrl each ganglion is con- 

 nected with the brain. The lirain is a pair of ganglia (Fig. 196, 

 cer. g) Ijing o^'er the oesophagus. It sends out ner\'es forward 

 to the various sense-organs of the head and backward to a 

 pair of ganglia lying just below the oesophagus and to a second 

 ]iair lying in the foot. These lower ganglia receive nerves from 

 the sldn and its muscles, and from the various xdscera. Thus 



