FRESH-WATER MUSSELS. 149 



ment it will die within a few days' time. In other words, the glochidium must pass the 

 life of a virtual parasite on the fish while undergoing its metamorphosis into the free- 

 living juvenile stage. In the light of our present knowledge, this is true of all the fresh- 

 water mussels (Unionidse) except the squaw-foot, Strophilus edcntulus, and one of the 

 small floaters, Anodonta imhecillis. The former species may complete its metamorphosis 

 either with or without parasitism (Lefevre and Curtis, 191 1 and 1912, p. 171; and 

 Howard, 1914, p. 44), while the latter, as it appears, never endures a condition of para- 

 sitism (Howard, 1914, p. 44). 



On coming in contact with the gill filament or fin of the fish the glochidium attaches 

 itself by firmly clamping its valves to the tissue of the host. A certain portion of the 

 tissue of the fish thus becomes inclosed within the mantle space of the glochidium, and this 

 quickly disintegrates and is taken into the cells of the glochidium and consumed as food 

 (Lefevre and Curtis, 191 2, p. 169). Within a very short time the tissue of the fish 

 commences to grow over the glochidium, presumably in an effort to heal the slight 

 wound caused by the "bite" of the glochidium, or perhaps as the result of a positive 

 stimulus imparted by the glochidium. L. B. Arey (report in preparation) successfully 



induced encystment by attaching to the 



filaments of excised gills of fish minute .••' 0^ """■■ ^-.,^_ 



metallic clamps the size of glochidia or / ,.f3|i?M>«iK ! I P 



smaller. The growth of tissue continues ,-.;• fr^-'r'':^ ''1 



until the larval mussel is completely •vTX"^- * '\ .i v^ - ' 



inclosed within a protective covering / • ^ > ' ^feiSpf- j. 



known as the cyst (PI. XVI, fig. 2). \ ^ ^.^ j, 



The several stages of encystment are ''• ^.AAjJ-'-i: ; /^O -' 



clearly represented in the series of fig- . . , 



■^ , Fig. 13. — Glochidium of pink heel-sputter, Lampsilts iProptera) 



UreS reproduced from Lefevre and Curtis alala, in condition of parasitism on giU of sheepshead, showing 



/•jQj2'\ Cpl XV fiff a) and the OrOCeSS growth of the juvenile mussel beyond the bounds of the glochidial 



- , . . - shell. 



may be completed withm 24or 36 hours. 



The appearance of a gill bearing a considerable number of glochidia is shown by 

 figure I of Plate XV, while figure 2 is an enlarged view of a few of the gill filaments of 

 a black bass carrying glochidia of the mucket. 



It is not our purpose to go in detail into the changes which occur in the glochidium 

 during the period of its parasitism. They are principally changes of internal structure 

 which scarcely affect the external appearance. Nevertheless, at the conclusion of para- 

 sitic life the young mussel is a very different sort of an organism from the simply organized 

 glochidium which has been described on page 143. Generally it has not increased in size, 

 but the single muscle v.'hich held the valves of the glochidial shell together has given place 

 to two adductor muscles as in the adult; the mouth and the intestine are formed, 

 the gills and foot are represented by rudiments which are prepared to function. The 

 larval mussel is, in fad , ready to begin its independent life and to take care of itself. All 

 of the changes which occur during parasitism require the expenditure of energy and the 

 use of body-building material, and as the glochidium enters upon the parasitic life with 

 no considerable store of food material, it is reasonable to assume that it derives at least 

 a small amount of nutritive material from the fish. Since no growth in size generally 

 occurs, the drain upon the fish therefore must be comparatively slight. There are, how- 

 ever, a few species (none of the commercial mussels, so far as we know) in which, during 

 the period of metamorphosis, the larval mussel grows to a comparatively large size 



