CULTURE OF FRESH-WATER MUSSELS. 79 



The gills are in the form of papillae, of which at this stage there are three or four 

 on each side of the foot, the longest being anterior, since it is the oldest or first developed 

 (see figs. 67 and 68). They are long, slender processes slightly recurved at the ends. 

 These increase in number with age and later become united to form the continuous 

 lamellae of the inner gill. The outer gills become visible between the first and second 

 month or at a length of between 3 and 5 millimeters. Schierholz's (1888) determina- 

 tion of the time as the second and third year for Anodonta and third and fourth for 

 Unio has been shown by Herbers (19 13) to be incorrect for Anodonta, and will probably 

 be found to be rather late for Unio. 



Other prominent features in the youngest juveniles are the liver and the adductor 

 muscles. The liver, because of its dark color, becomes quite prominent before the 

 young mussel leaves the fish. It furnishes in this manner a ready index for the degree 

 of development when examined alive. The adductor muscles also become conspicuous, 

 but in another way. Because of their form and an index of refraction higher than that 

 of the surrounding tissues they appear as bright spots. The stomach and intestines 

 seem to become functional at once, the latter at first with a few turns comes graduallv 

 to the tortuous condition in the adult. The heart and kidney can not readily be made 

 out in whole mounts. Herbers (19 13) by sectioning finds their development pretty 

 well advanced in Anodonta cclensis at a length of 2.59 millimeters, corresponding to the 

 second month in Lampsilis luteola. 



The mantle is a direct derivative of the same organ in the glochidium. The coming 

 of free life marks a change in its function. Where in the glochidial and parasitic periods 

 (in this species) no increase of shell occurs, in the juvenile stage a phenomenal growth 

 takes place. Beginning as a delicate microscopic membrane lining the glochidial shell, 

 it increases with the growth of the mussel until, as we have seen, it is increased in size 

 thousands of times in a single summer and eventually produces the heavy shell, the 

 protective armor of the grown mussel. 



The shell of juveniles up to the second month has two features that are characteristic 

 of this early period. In consistency it is like horn, being transparent and less hard than 

 later, when it becomes calcareous. The surface is uneven owing to a series of regular 

 and relatively high undulations, knobs, etc., which are characteristic for each species 

 (fig. 73). These are designated as "umbonal sculptures" by conchologists in describing 

 the adult mussel, in which they are not infrequently found well preserved. 



A structure to which special attention is called is the byssus, an organ that is charac- 

 teristic of the juvenile stage in certain groups of fresh-water mussels. It consists of a 

 hyaline thread produced by the byssus gland located on the ventral and posterior 

 median edge of the foot. The first instance of it observed in the present culture was at 

 an age of about 38 days, when the smallest of the mussels collected had a length of 

 4 millimeters (other cultures 1.9 millimeters). In this same species in nature the author 

 has seen it present at a size of 2.8 millimeters. In juveniles of Ouadrula heros, at an 

 age of a few days, there is apparent a tough mucous-like secretion that serves to anchor 

 the young mussel. Near the end of the growing season byssi were found on mussels of 

 over 1 inch in length. The strength and caliber of the threads are appropriate to the 

 size of the mussel. When the mussels were removed from the water at a temperature 

 near that of freezing on November 20, attachment by byssi was not noted. However, 



