FRESH- WATER MUSSELS. 157 



of the glochidium is accomplished at the expense of the parent rather than of a fish. 

 Howard (1915) subsequently found that the glochidia of this species could be made to 

 attach to fish and would undergo metamorphosis in the usual way on this fish. He also 

 discovered that tlie glochidia of another species, a small floater, Aiwdonla imbecilHs, 

 developed into the juvenile mussel within the gills of the parent, and that they would not 

 remain attached to fish. 



It is significant that there are just a few species of mussels which diverge in two 

 directions from the general rule that fresh-water mussels undergo metamorphosis only in 

 parasitism and without evident growth in size during the process. On the one hand, 

 we have the cases just cited of change of form accomplished without parasitism, and on 

 the other the instances mentioned on page 149 of two or three species in which the larval 

 mussel increases many times in growth while still encysted upon the fish. The tendency 

 manifested by two sjjecies is toward independence of fishes or other hosts, while the 

 tendency revealed by a few others is toward a much greater dependence upon fishes. 

 The vast majority of species, including all the mussels having shells of commercial value,* 

 occupy the middle ground of limited dependence upon fish; they must live upon the 

 fish, but they require little from them. The hope has been cherished that in time a 

 means would be found of supplying artificially to the glochidia of the common species of 

 useful mussels the food materials and other conditions necessary for the metamorphosis. 

 so that it might become possible to rear mussels without the use of fish. So far, how- 

 ever, failure has marked every attempt to accomplish this purpose. 



JUVENILE STAGE. 



At the close of the period of parasite life, the young mussel is no longer a glochidium, 

 and while it possesses the rudiments of the principal organs of the adult, it has yet to 

 undergo many changes of structure — or better perhaps, a progressive development in 

 structure — before it fully assumes the adult form and manner of life (PI. XV, figs. 5 and 6; 

 PI. XVII, fig. 4). To the intennediate stages, or series of stages, between parasitism and 

 the development of functional sex organs the term juvenile may properly be applied. 

 The siphons or respiratory tubes, the labial palps, outer gills, and sex glands are among 

 the conspicuous features of structure acquired during this stage. 



With many and probably most of the common species of mussels, the early juve- 

 nile mussel is no larger than the glochidium — in the case of the Lake Pepin mucket shghtly 

 less than one one-hundredth inch in length and slightly more than one one-hundredth inch 

 in height. Its thin mussel shell underlies the glochidial shell, and is scarcely visible until 

 after several days of growth. The most conspicuous feature of the young mussel at this 

 time is the foot, which may be protruded from the shell as a relatively long, slender, and 

 active organ of locomotion. The following description applies primarily to the Lake 

 Pepin mucket : The foot is somewhat cleft at the apex to give a bilobed appearance and 

 it is clothed with cilia or minute living paddles, which are in rapid motion while the foot 

 is extended. The foot has also the power of adhesion to surfaces as smooth as glass; by 

 means of it the young mussel can move about rapidl)' or eff'ect temporary attachments to 

 foreign objects. It is not long before the peculiar characters of the juvenile foot are lost, 

 for during the first month of independent life this organ becomes changed into the char- 

 acteristic form of the foot of the adult mussel. 

 75412'=— 22 11 



