STANDEN : THE ZEBRA-MUSSEL. 89 



Tig. 5. 



Fig. 5. — Dorsal view of young Dreissensia crawling, with foot {f) much protruded (from 

 Korschelt, '92). 



By this time the Httle mussel is provided with an almost circular 

 shell, which might be taken for a young Sphceriian, and as it increases 

 in size the growth of the foot is retarded, and gradually acquires the 

 stumpy form possessed by the adult. The shell finally becomes fixed 

 but still retains a certain amount of mobility, in that, under certain 

 circumstances, e.g., scarcity of water, it is able to detach itself from its 

 base and move slowly to another spot, and during the winter it 

 hibernates, casting off its byssus, and retiring under the mud in deep 

 water. 



There is not the slightest doubt that in the different phases of 

 mobile power exhibited by Dreissetisia during the early stages of its 

 existence, we have the correct explanation of its astonishingly rapid 

 dispersal. The abiUty of the larvse to rove about, and the ease with 

 which they can be carried to a long distance by currents, give them a 

 great advantage over other freshwater bivalves. 



In its more adult stages its tenacity of life, adaptability to circum- 

 stances, unusually rapid propagation, the power of attachment by a 

 strong byssus to rafts of timber floating from place to place, cr to 

 bottoms of boats — thus enabling it to advance up streams— together 

 with other facilities for dispersal quite independent of man's agency, 

 have combined to make it one of the most successful molluscan 

 colonists in the world. Altogether it is gratifying to think that Dr. 

 Korschelt has, by his valuable discoveries, at last raised the veil of 

 mystery which had previously hidden an unsuspected feature in 

 the life-history of this shell, and helped materially to solve what was 

 for long a profound puzzle to conchologists past and present. 



The following is a list of all the local, and the principal general 

 literature connected with the subject that has come under my notice: — - 

 '25 SOWERBY, J. DE C, [November 2, 1824, presentation of specimen of a fresh- 

 water shell, probably the Mytilus polyntorphus Gmel, 3363]. Trans. Linn, 

 Sec, vol. 14, p. 585, 1825. 



