284 W. J. CROZIER 



4) The number of eggs contained in a single normal egg 

 ribbon is proportional to the size of the animal. 



These facts lead to the view that assortive mating is distinctly 

 advantageous to C. zebra. It results in the conservation of eggs 

 (presumably of spemi also), and, through the couphng of large 

 individuals, insures that the number of fertihzed eggs actually 

 set free shall be greater than that which would follow from 

 random pairing. Both of these effects may be regarded as 

 making more certain the establishment of the greatest possible 

 number of larvae. 



The adult C. zebra, as I have elsewhere described (Crozier, 

 '16b; '17 a), is well provided with the means for securing im- 

 munity from destructive attack. This opinion is not vitiated, 

 but is on the contrary emphatically substantiated, by the oc- 

 currence of injured specimens, as previously noted in this paper. 

 This ni^dibranch is in fact one of the most plentiful animals in 

 Bermuda waters. That the pelagic veliger is equally immune, 

 seems highly improbable. For the continuance and increase of 

 the species the larvae must be as numerous as possible, and 

 toward this end many features in the ecology of Chromodoris 

 cooperate. From April to June the eggs require 15 to 20 days 

 (in the field) before they escape from the ribbon of jelly and 

 lead for a time a free swimming existence; this is about the 

 period recorded by other observers for the development to hatch- 

 ing in various Doris eggs (Pelseneer, '99, p. 515, where the hatch- 

 ing time of Eolis eggs is given as 9-17 days; and Eliot, '10). 

 During this early developmental period, the eggs are pro- 

 tected by a repugnant jelly. I have never found an egg mass in 

 nature which had been damaged, and no animals will partake of 

 them, although they are commonly deposited in exposed situa- 

 tions, and are of a bright red color (becoming pale orange as 

 development proceeds). Among the notable features of this 

 case the following may be mentioned: 1) reproduction through- 

 out the year; 2) the shoreward migrations of many individuals 

 at certain periods, and the responses which at such times result 

 in the concentration of the nudibranchs within the boundaries 

 of creeks and bays, so that large numbers (sometimes hundreds) 



