6S 



BULLETIN OF THE BUREAU OF FISHERIES. 



the conditions severe for the fish and killing five of the eight. On June 10, 20 days from 

 the date of infection, most of the young mussels were found to have been shed from the 

 three remaining fish. On the same date shedding was found to have taken place from 

 infected fish placed in the cement ponds and aquaria. The time of shedding for the 

 earth ponds was not observed. 



The young mussels were secured at this early stage from the aquaria. At the time 

 of shedding there is apparently no growth of shell beyond that of the original glochidium, 

 but the young mussel (see fig. 67) internally has for the most part the organs of the adult 

 in contrast with the simple structure of the larval glochidium. Growth of the shell 

 begins at once (see figs. 67 and 68), as shown, and in the figure a narrow border of the 

 new shell is already visible. 



GROWTH IN FLOATING CRATES. 



Two weeks after obtaining the plant of young mussels from the bass, evidence that 

 they were thriving in the crate was obtained. A small sample of sediment from the 

 bottom revealed some half dozen or more. These had already a considerable growth 



of shell, the largest having an increase 

 in surface of at least three times the 

 size of the original glochidium (see 

 fig. 69). 



At various intervals throughout the 

 summer and autumn the author readily 

 obtained specimens, making observa- 

 tions on rate of growth and preparing 

 material for studies of development. 

 Figure 66 shows individuals illustrat- 

 ing the amount of growth from month 

 to month. The last examination was 

 made about November 20, when the 

 whole plant in the basket (fig. 75) 

 was photographed under water. Later 

 they were removed from the mud, a census was taken, and more photographs were 

 made (fig. 70) . After completing such observations as were feasible upon the whole plant 

 of living mussels they were returned to a crate and placed in a pond to spend the winter. 

 The series shown in figure 66 represents about the average ' growth from month 

 to month. These, with the exception of the third, were removed from the basket on 

 the dates given in Table 1, page 69. By inspection it is obvious that the rate of increase 

 in growth as represented by these is not uniform throughout. This is due partly to 

 the fact that in some cases small numbers only were removed at a time. In this way 

 the average size was not secured in each instance. In one case only was a voluntary 

 selection possible, and this was the last, made from several of nearly equal size. The 

 specimen in the series for the second month (fig. 66, second from left) was probably 

 smaller than the average. It will be noted that by months the increase is much more 

 rapid at first, so that the rate is a decreasing one. 



Fig. 69. — A juvenile mussel 15 days after the beginning of free-living 

 stage, or about two weeks older than that of figure 68. View of 

 right side. Drawn with a camera lucida. X 140. 



1 These were selected at random in most cases and so probably approximate the average. 



