378 THE AMERICAN NATURALIST [VOL. LI 



noticed that a certain species of nudibranch, until then found 

 sparsely, if at all, suddenly begins to occur in abundance. It is 

 also observed that at this time these nudibranchs are depositing 

 eggs in the field, or that they pair readily and lay egg strings 

 when kept in aquaria. The inference which has been drawn in 

 such cases, namely, that the appearance in shoal water is in some 

 way intimately related to the mating process, seems legitimate 

 enough. 



But I have observed at Bermuda certain facts regarding the 

 normal migrations of a member of the typically tropical genus 

 Chromodoris which, it seems to me, cast considerable doubt on 

 the theory that this species, C. zebra Heilprin, moves into shal- 

 low water for the purposes of mating and egg deposition. The 

 facts in this case, so far as they have been observed, are briefly 

 as follows: 



It was necessary to obtain considerable numbers of C. zebra 

 for use in experimental work (Crozier, 1916% 1916 b ) ; conse- 

 quently collections were made at short intervals (every day 

 during some months) over the period from August, 1915, to 

 October, 1916. I had had occasion, also, to note the occurrence 

 of this species in the summers of 1913 and 1914. In June, and 

 during the early part of July, Chromodoris was found in great 

 abundance upon the "eel grass" in certain tidal "creeks" 

 (Fairyland Creek, Millbrook Creek). Subsequently, in the last 

 two weeks of July and in August, they became very scarce in 

 such places, although a few could almost always be discovered 

 by careful searching. At other times of the year a supply of 

 the animals was obtained on hard, open bottoms in somewhat 

 deeper water (1 to 2 fathoms, at low tide), in places where, I 

 am certain, they would never have been seen during ordinary 

 shore collecting. Occasionally, however, as was noted particu- 

 larly in December, 1915, Chromodoris was abundant along the 

 rocky shores of smaller islands, ranging well up to low-water 

 level. 



It seems clear enough that in Chromodoris zebra there un- 

 doubtedly does occur from time to time a movement of numbers 

 of individuals toward the shore. But there are several facts 

 which sharply contradict the view that this migration is con- 

 nected with reproduction. The nudibranchs pair in the lab- 

 oratory and lay strings of fertile eggs at all seasons of the year 

 (cf. also Smallwood, 1910), and not merely at the times when 



