94 JOURNAL, BOMBA Y NATURAL HISTORY SOCIETY, Vol. XVIII. 



Life Histories. 



Among the Gasteropoda, one of the largest families are the Limnceidm or 

 water-snails as they are commonly called, and are found in more or less abun- 

 dance in all ponds, lakes, and river?. Like land-snails they breathe air and 

 frequently come to the surface to obtain this. Other species of Gasteropoda 

 on the other hand breathe dissolved oxygen by means of gills which are 

 situated beneath the shell. 



The Limnceidce are hermaphrodite, but "the male and female apertures are 

 separate from one another, the latter being situated beneath the mantle flap 

 some distance from the former, which is placed as in Helix"* The eggs of 

 Limncea are deposited in long jelly-like masses and fastened to water weeds. 

 In aquaria Limnma will be found very useful devouring the green scum that so 

 thickly adheres to the sides, and they moreover help to keep the water fresh 

 and pure. 



Unio may be taken as a typical bivalve, and the life history of this is 

 extremely curious and interesting. If an ordinary pond mussel be examined 

 closely when at rest in the water, at the anterior part of the shell the mantle 

 folds will be seen slightly projecting their edges forming two short tubes. 

 These are known as the inhalant syphon, and it is through this the animal 

 breathes and feeds. Immediately above the inhalant syphon will be seen a 

 short tube known as the exhalant syphon through which all waste matter is 

 expelled. The large fleshy organ protruding from the front of the shell ia 

 known as the " foot " by means of which the animal moves and buries itself in 

 the mud. It has been roughly estimated that the rate of progress in a year is 

 one mile. Mussels are extremely sensitive to vibrations, and if some be kept 

 in a bowl a heavy foot-fall near at hand is sufficient to cause every individual 

 to immediately close its shell, and this is due to certain powerful muscles 

 which will be found on dissection adhering to the shell, and indeed so tightly 

 are the valves closed as to render the opening of a live mussel an impossibility 

 without breaking the shell. The sexes are separate, rarely hermaphrodite, and 

 it is said females are more abundant than males. There is no union of sexes. 

 The spermatozoa of the male is discharged into the water and carried by ciliary 

 currents into the inhalant syphon of the female. The eggs pass into the gill 

 plates which serve as brood pouches. Young mussels when first hatched fix 

 themselves to the tail or fin of a fish and then form a cyst which completely 

 covers them, and thus for a time live as a parasite feeding on the tissues of 

 their host. Eventually the cyst withers and the young mussel drops off to 

 lead an independent life like its parent. 



Economic Uses. 

 As an article of diet, freshwater mollusca are of little or no importance to 

 mankind. Their shells, however, are used extensively for the manufac- 

 ture of lime, and the gathering of them for this purpose is an industry carried 

 on in many parts of India. Some of the Uttios produce pearls of some value. 

 * Latter. Natural History of Some Common Animals. 



