Invertebrate Gallery of the Indian Museum. 139 



to make the most surprising leaps. In a few forms — e.^.^ 

 the Razor-shells — it is a long pointed cylinder used for 

 boring in the sand. In many sessile forms—^.^., the true 

 Mussels — the foot is provided with glands, known collect- 

 ively as the "byssus gland", the secretion of which, tough- 

 ening on exposure to water, forms a bunch of silky 

 threads — the byssus — by means of which the animal anchors 

 or permanently attaches itself. In the Oysters the foot is 

 altogether absent. 



Bivalves are all aquatic, and though there is a large 

 number of nearly similar freshwater forms, the great bulk 

 of the Class is marine, the species occurring at all depths 

 and in all situations. 



Their typical habit of life may most easily be studied 

 in the Freshwater Mussel. In repose this animal lies half 

 buried, front downwards, in the mud, with the posterior 

 end, where the "siphons " open, projecting into the over- 

 lying water, and the valves of the shell slightly gaping. 

 Currents of water, set agoing by the "cilia" which line 

 the mantle-chamber and gills, and carrying air and the 

 microscopic particles on which the animal lives, pass into 

 the mantle-chamber at the incurrent siphon and sweeping 

 the gills, where they exchange their oxygen for carbonic 

 acid, reach the mouth, where they deliver up their burden 

 of nutriment. Ultimately they pass out at the excurrent 

 siphon sweeping the vent as they go. 



The marine Bivalves live either buried in the mud or 

 firmly attached to rocks, etc : many, however, are able to 

 crawl or hop by means of their muscular foot, and some swim 

 with some activity by opening and closing the valves of 

 the shell, like wings. The group of Pholads and Teredos 

 live deeply buried in rock, submerged wood, etc., which 

 they excavate by rotating their often specially hardened 

 shells. The Razor-shells burrow deep in the sand. 



Many Bivalves are able to form pearls, which are only 

 derivations of the iridescent pearly layer (" mother-of- 

 pearl,'' or nacre) that lines the shells not only of Bivalves 



