380 THE AMERICAN NATURALIST [VOL. LI 



fact of egg production throughout the year in deep water is, 

 however, good evidence that the periodic (or intermittent) 

 abundance of this nudibranch in shoal situations can have little 

 if any relation to oviposition. 



It might at first be suspected that the periodic shoreward 

 movement represents the phylogenetic persistence of a well- 

 defined habit possessed by not distantly related northern species. 

 From this standpoint, reproductive functions in C. zebra might 

 be conceived of as having become dissociated from the habit of 

 migratory periodicity, since in warmer seas, where the seasonal 

 alteration in physical conditions is reduced to a minimum, it is 

 well known (cf., for example, Semper, 1881, p. 135) that many 

 forms have no specially restricted time for breeding. How- 

 ever attractive such a speculation may appear, it is eminently 

 more satisfactory to regard these periodic littoral appearances 

 of tropical nudibranchs as being controlled by definite physical 

 influences in each individual case. Such directing causes would 

 not necessarily be always the same for each periodic occurrence 

 of the animals in shallow water. Although shoreward migra- 

 tion and egg laying are closely connected in northern forms, it 

 is still probable that physical circumstances in the sea imme- 

 diately control the migrations even in this instance also. 



I have purposely refrained until now from discussing certain 

 minor fluctuations in the littoral abundance of Chromodoris 

 which are, nevertheless, important in connection with the idea 

 that the supposed ; ' migration ' ' at certain times into very shallow 

 water is, after all, only the unrestricted expression of a tendency 

 to upward movement negative geotropism. It has been men- 

 tioned that during the greater part of the year Chromodoris 

 was collected in 1 to 2 fathoms. But after storms of some 

 severity they were to be had only in much deeper water. The 

 nudibranchs undoubtedly move into deep, quiet places when the 

 surface is greatly disturbed. Just what their behavior is under 

 these circumstances can not be stated from direct observation, 

 for obvious reasons; and for several days, or even for a week 

 after a severe blow, the water in the sounds and bays remains 

 so roily that it is impossible to see the bottom. But I have fre- 

 quently observed individuals creeping up from deep water after 

 the sea has become quiet and transparent. As regards the bear- 

 ing of these facts upon the major flocking into the littoral zone, 

 which occurs in early summer, it is to be noted that the mere 



