200 BULLETIN OK THE BUREAU OF FISHERIES. 



be seen in total preparations rendered transparent, which led to the conclusion that in 

 the interval between the 0.512 mm. and the 0.72 mm. stages a sudden change in the 

 activity of these cells occurs which results in a rapid proliferation to form the series of 

 reproductive canals that ramify throughout the body tissue. This latter condition was 

 observed in sections of specimens 1 mm. long. 



The age at which the various organs appear during the later metamorphosis can be 

 stated only approximately, since it was impossible to carry normally developing embryos 

 beyond the first week. All the stages represented beyond figure 1831 (p. 193) were drawn 

 from specimens 3 collected in Casco Bay near Harpswell, Me., during the month of 

 August, which offers no clue as to age, since the reproductive process is more or less con- 

 tinuous throughout the summer. Matthews (1913), however, was able to rear mussel 

 larvae in the laboratory by feeding them on cultures of Nitzachia, and she succeeded in 

 keeping them alive for months. Metamorphosis in these artificially reared mussels 

 appears to have taken place more rapidly in proportion to increase in size than in nor- 

 mally grown specimens. The artificially reared larvae, for example, measured 0.31 by 

 0.24 mm. when at the five-gill filament stage, whereas normal larvae of the same stage 

 measure 0.385 by 0.320 mm. On the basis of Matthews's observations it is probable that 

 the stage represented in figure 184 (p. 197) is approximately 6 weeks old; that in figure 

 186 (p. 197), 2 months old; while that in figure 188 (p. 198) is not more than 10 weeks old. 



GROWTH. 



The rate of growth which takes place in the mussel after it reaches the attachment 

 stage depends upon several factors, the chief one of which is abundance of food. If 

 food is scarce, growth is retarded regardless of all other conditions. On the other hand, 

 if diatoms, Protozoa, and spores of algae are abundant in the water which flows over the 

 beds and at the same time mud, sand, and filamentous algae are absent, growth will 

 take place rapidly. These conditions are further influenced by the rate and volume of 

 the currents flowing over the beds and by the length of time the mussels are exposed to 

 the air during each tide. Targe volumes of water moving slowly supply food most 

 advantageously to the mollusk, and where the beds are not exposed the food is contin- 

 uously brought to the shellfish without interruption. For this reason the largest and 

 best mussels are found in beds where the water covers them to a depth of from 6 

 to 15 feet. 



Salinity of the water is thought by some observers to influence the rate of growth. 

 Brandt (1897) noted that in Kielwight the mussel grows to a length of 4^ inches, while 

 in the Gulf of Bothnia, where the salinity of the water is less, the mussel only attains a 

 size of about 1 inch. In the Kaiser Wilhelrn Canal, where the salinity of the water 

 decreases from east to west, he found that the ripe mussels in the fresher parts were only 

 about half the size of those growing in the saltier regions. 



Under ideal conditions the mussel will increase about an inch in length annually 

 for the first two or three years, and then the rate of growth gradually diminishes. Mus- 

 sels, however, do not frequently find such situations, so that the average rate of growth 

 as actually found on the natural beds is much less. Ordinarily tne time required for 

 ■the shellfish to att ain a lengthof 3 inches is five to seven years. 



8 The author is indebted to Dr. Edward L. Rice, of Ohio Wesleyan University, for these specimens. 



