Xviii INTEODUCTIOtf. 



this being accomplished by a sharp contraction of the pedal muscle, 

 the shell and the remainder of the animal are forced suddenly 

 forward. 



Both Gastropods and Pelecypods, however, are not dependent 

 on the foot only for voluntary movement, among the former 

 Limncea especially, and among the latter Pisidium are able to put 

 forth filaments of mucous by which they are enabled to ascend 

 and descend to and from the surface of the water and not only to 

 float on, but to skim along it, foot uppermost. 



The more or less artificial dispersal of mollusca is a subject 

 upon which, from time to time, a good deal has been written ; 

 undoubtedly the larval Gasteropod forms are often swept many 

 miles hy floods, frequently by this means being stranded in pools 

 and ponds isolated in normal times and to which it would 

 necessarily be impossible for them to obtain access were it not for 

 the agency of waterfowl which, there can be little doubt, do carry 

 certain species about in the plumage just above the legs, con- 

 siderable opportunity being given for the mollusca to ascend these 

 and become entangled in the feathers during the lengthy periods 

 in which the birds remain stationary in the water watching for 

 their prey. 



The Pelecypoda also are great travellers, but this is due to their 

 being carried far in their embryonic state by the hosts to which 

 they have attached themselves ; once the parasitic larval stage is 

 over, they are seldom able, except through an exceptional accident, 

 to move far afield. 



Prom the foregoing remarks, however, it must not by any means 

 be assumed that all the fluviatile species are widely distributed, 

 many instances occur of extremely localized forms, among which 

 may be mentioned as examples the Limncea involuta of Killarney 

 and some of the peculiar forms of Lake Tanganyika which do not 

 seem to have become diffused in the remainder of the Congo basin. 



6. Economic Uses. The purification of water is doubtless to 

 a certain extent performed by the mollusca in their capacity of 

 scavengers, in addition to which they also provide food for valu- 

 able birds and fish ; as a rule, however, the fluviatile species are 

 not in much request as food for man ; though in Guadeloupe, 

 Mauritius, China, Japan, and even in France, certain species of 

 freshwater bivalves are* used for human consumption. 



The chief services rendered to the human race are, however, 

 rather ornamental than actually useful; in the Unites States 

 certain species of UnionidcR are extensively fished and even culti- 

 vated for their nacre which is used largely for button punching, 

 small knife handles and other purposes to which mother-o'-pearl 

 is put, while as a bi-product pearls themselves are doubtless 

 obtained ; indeed, a case has recently been placed on record of as 

 many as nine hundred and twelve being taken from a single 

 individual *, one hundred of which were of marketable size, though 



* Nantilus, Boston, Mass., xxv.. p. 84. 



