414 ANIMAL LIFE-HISTORIES 



which is a loosely wound spiral tube with only the first few turns 

 in contact. 



Among air-breathing Freshwater Snails the spawn is usually 

 gelatinous, common examples being afforded by the Pond-Snails 

 iyLimncsa), where the mass is elongated and sausage - shaped, 

 and the Trumpet- Snails {P/anorbis), in which it is oval. These 

 collections of eggs are attached to water-plants, or, in captive 

 specimens, are often fixed to the glass walls of the aquarium. 

 A somewhat exceptional case is presented by our common River- 

 Snail {Paludma vivipard), in which the eggs are not laid, but 

 develop internally, and the young snails when they first enter the 

 world resemble the adult except in size. 



Of terrestrial forms many of the Land-Slugs (e.g. species of 

 Limax) deposit gelatinous masses of spawn, in which numerous 

 eggs are imbedded. Land- Snails lay a varying number of eggs, 

 each enclosed in a more or less firm calcareous shell. The 

 common Garden-Snail i^Helix aspe7^sd) digs a little hole in the 

 earth, deposits from 40 to 100 eggs in this, and then covers 

 them over with soil. Some of the large South American Land- 

 Snails (species of Bulimiis) lay very large eggs, which super- 

 ficially resemble those of a pigeon, but have a widely different 

 structure, for the included egg-cell is minute, most of the space 

 within the shell being filled with a nutritive fluid. These eggs 

 are deposited singly, and in the case of the arboreal species each 

 of them is enclosed in a rolled-up leaf 



Gastropod Larvcs. — As in bivalves, the eggs of marine forms 

 usually hatch out as free-swimming larvae, which may be either 

 trochospheres or veligers. These vary greatly in appearance, 

 as may be gathered from fig. 940, in which a few typical forms 

 are represented. Development is direct in freshwater and ter- 

 restrial species, there being no free - swimming larva, though a 

 stage corresponding to it may be passed through before hatching 

 takes place, proving that development was indirect in ancestral 

 forms, and that the existing state of things is an adaptation to 

 changed surroundings. In Onchidium, for example, a curious 

 slug which lives on the upper part of the shore in various parts 

 of the world, a typical veliger is developed within the ^<g'g (fig. 

 940), but this is gradually modified into the adult shape before 

 the time of hatching. 



Gasti'opod Dwellings. — Permanent homes constructed by the 



