164 PEESH-WATBE AQUARIA. 



distinct, but as they grow older, each individual becomes both 

 male and female. 



5. Some snails are oviparous, and others ovovivipa/rous (the 

 eggs being hatched within the oviduct of the parent). 



6. Some snails are puhnobranchiate (possessors of lung-like 

 branchiae for breathing the atmospheric air), and others are 

 pectini-hrancMate (possessors of comb-like branchiae, adapted 

 for respiration beneath the water). 



7. Some snails have their eyes placed upon short footstalks 

 at the base of their tentacles, while others have theirs sessile 

 there. The tentacles of all the species are situated above 

 the mouth, and are used for touching, and perhaps also for 

 smelling. 



The oviparous animals, according to their different species, 

 deposit their ova in masses, which are in shape either 

 cylindrical or elliptical, or orbicular or round. The eggs in 

 some of the capsules are as few as from three to six; 

 whUe in others they are as many as from 80 to 130. They 

 are affixed either to stones, or to leaves or stalks of water- 

 plants, or sometimes to the shells of one another. The eggs 

 are hatched, according to the different species, in from ten 

 to about twenty-six days. 



Many water-snails, perhaps nearly all, have the power 

 of " spinning " a kind of gelatinous thread by means of 

 which they are able to ascend or descend in the water. Mr. 

 Gr. Sherriff Tye, the writer of an interesting article on " Mol- 

 luscan Threads," which was pubhshed in Science Gossip 

 for March, 1874, in speaking of the habit of the Physa 

 hypnorum as a " thread- spinner " says: "One of the most 

 beautiful sights in molluscan economy is to see these little 

 ' golden pippins ' gliding through the water by no visible 

 means; and when they fight, to see them twist and twirl, 

 performing such quick and curious evolutions, while seemingly 

 floating in mid-water, is astonishing, even to the patient 

 student of Nature's wonders.'' These threads are anchored 

 to the surface of the water in a curious way, viz., "a 

 minute concavity at the upper end acts like a small boat of 

 air, and thus sustains the thread." 



