STUDIES ON SEX IN CREPIDULA 7 



compared with those kept at Woods Hole during the summer 

 months. For instance, about fifty very small specimens (2 to 3 

 mm. in length) were taken from a consignment sent from Woods 

 Hole to Princeton in November, 1915, and placed in a tumbler 

 in the salt water aquarium. During five months following they 

 showed hardly any growth at all. At the last of March no speci- 

 men in the tumbler was more than 4 mm. in length. A similar 

 lack of growth was observed in larger specimens. At Woods 

 Hole in the summer swimming larvae settled on some of the 

 hermit shells in the float car and grew to considerable size, 

 some up to 10 to 15 mm. length, within two months. How 

 much of the difference in amount of growth in the two situations 

 ought to be ascribed to the season of year will not be perfectly 

 clear until the animals have been studied during the winter in 

 their natural environment. It was noted, however, in June, 

 1916, that many of the specimens wintered in the vivarium with- 

 out growing were beginniag to exhibit a considerable margin of 

 new shell, and it is probable that there is a marked seasonal dif- 

 ference in normal growth. Indeed this must be the case; for if 

 growth went on as rapidly during the winter as in the summer^ 

 very few small specimens could be found in the spring, since the 

 production of eggs does not continue to any extent during cold 

 weather; but many small individuals can be found in the spring. 

 The shape and color of the shell indicates clearly that they are 

 not young, but belong to a brood of the previous autumn. 

 Their growth has been retarded during the winter. 



This ready response of the organism to the environment in the 

 matter of growth is one of the conditions which so greatly com- 

 plicate the study of the sexual cycle. In the large and crowded 

 colonies, where many individuals are wedged in between and 

 lying upon one another, possible dwarfing is so confusing a 

 factor that it is necessary almost to disregard size when selecting 

 material. 



Summing up the facts already discussed in regard to growth, 

 it may be said: 1) Apparently no rapid growth of the body of 

 Crepidula plana can take place without free extension of the 

 mantle and the deposition of new shell, a) Frequent movement 



