296 ANIMAL AGGREGATIONS 



The variability of the male phase is summarized by Gould (1917a) 

 as follows: ''A number of specimens of the same size and apparently 

 of the same age, taken at the same time of year, may show widely 

 different sexual states. One may be a fully developed male; one 

 may exhibit evidence of having been a male, though the male 

 characters are being lost; and one may furnish no suggestion that 

 any male characters have ever developed." 



An analysis of these conditions shows that the development of 

 the male phase is dependent on the nearby presence of a larger 

 individual of the same species, which is usually, though not neces- 

 sarily, a female. The greater the difference in size between the 

 smaller and the larger animal, the more certain and complete is 

 the development of the smaller in the male direction. A greater 

 stimulus is necessary to complete male development than to initiate 

 it. 



In nature these snails develop from free-swimming veliger larvae 

 which settle within the gasteropod shells occupied by hermit crabs. 

 If the snail which has formed the shell has recently died, the Crepidu- 

 la population will be small; but in the shells long occupied by the 

 crabs, a large number of Crepidula may collect. Those located near 

 the outer margin will show little sign of movement. Their own shells 

 will have grown to fit the irregularities of the substratum. Usually 

 these are large females. Crowded about them, occupying vacant 

 patches, and back in the deeper recesses of the shell, the Crepidula 

 are smaller; and, since their shells are not intimately fitted to their 

 surroundings, it may be assumed that they move about. 



If a larval Crepidula plana settles into a shell where no larger 

 members of its species are present, the male phase is normally 

 omitted. In cases where Gould found neuters and returned them 

 to hermit crab shells free from large individuals,- he recovered at the 

 end of 34 days 24 specimens, which, when sectioned, showed 3 with 

 adult testis, 2 with spermatids, i with spermatocytes, 1 1 with sper- 

 matogonia only (that is, no further development toward sperma- 

 tozoa), 5 sexually inactive, and 2 with oogonia. Of 2 specimens 

 found to be neuters, isolated on hermit crab shells and sectioned 

 after 67 days, i had early oocytes and the other had ova with yolk. 



