DISPERSAL BY REPTILES 183 



Neritina, which belong to the Streptoneura. None 

 of them pass through a free swimming stage, as the 

 majority of their marine relations do. The Euthyneura 

 usually deposit their eggs in a jelly, which is attached to 

 some foreign object, and the young emerge in a fully- 

 formed state. Of the Streptoneura, Bithynia attaches 

 its eggs to foreign objects, but Paludina is viviparous, and 

 the young leave the mantle chamber of the parent as 

 fully-formed adults. Since the univalves enjoy the 

 power of creeping about, there is less necessity for 

 devices to secure dispersal of the young, though the 



FIG. 55. Paludina vivipara, Linn., a" 

 Freshwater Snail, which carries its 

 eggs within its shell. Natural size. 

 From Bronn's " Thierreichs." 



attachment of the eggs may lead to this, for they are 

 deposited on any surface that lies handy stones, the 

 leaves of plants, or the shells of other snails ; and in 

 ancient times, when crocodiles and tortoises were 

 common inhabitants of our rivers, the hard surface of 

 their scutes or armour would have suited the purpose 

 admirably. Even the adult snail may get a lift on the 

 back of one of these reptiles. A species of Limnaea has 

 been seen riding on the backs of turtles which were kept 

 in a pond near Belfast ; and it would be strange if croco- 

 diles and alligators, which lie like logs in the waters they 

 infest, but travel swiftly enough in their annual migra- 



