NOTES ON SOME PROBLEMS OF ADAPTATION. IO7 



2. Owing to the nature of its normal mode of occurrence, 

 which is most frequently in bright, sunlit situations in shallow 

 water, A. atra is rendered quite conspicuous by its dense blue- 

 black coloration so much so, in fact, that some observers (cf. 

 Mortensen, '17) have not hesitated to suggest the operation of 

 "warning" coloration in this connection. In view of this fact, 

 and of the direct relation, in Ascidia, between pigmentation and 

 blood metabolism (Hecht, 'i8a), the finding of pale individuals 

 in dark situations required further analysis. 



Simple ascidians having transparent or translucent bodies are 

 commonly found to frequent dimly illuminated spots. Examples 

 are given by Ascidia cttrvata, quite transparent and at Bermuda 

 found almost always under fair-sized stone slabs, although taken 

 by me also among masses of the alga Valonia, the dead cells of 

 which are closely imitated by it (!); by A. calif ornica (Ritter 

 and Forsyth, '17), from the undersides of rocks; and by Ciona 

 intestinalis, "especially given to clinging to the undersides of 

 floats, buoys, and the like." The original assumption of a 

 darkened habitat by these forms is probably due to the fact thar 

 the ascidian larva, at first photopositive, becomes sooner or latet 

 negatively heliotropic, before attaching; in different species this 

 reversal of behavior with respect to light may be much delayed, 

 or may even not appear at all. (In another place I shall discuss 

 experiments concerned with the behavior of the tunicate tadpole.) 

 It is of interest to note that the adult Ciona is quite reactive to 

 light, in a sensory way, and also that in the laboratory these 

 animals die rather quickly in diffuse light, although in dim light 

 or darkness they are able to live for some days (Hecht, 'i8a); 

 whereas A. atra, adult, is insensitive to photic excitation (Hecht, 

 'i8b; Day, '19). 



Very pale examples of A. atra, however, were, as stated, found 

 under stones in various places at Bermuda. Some of these were 

 carefully gathered, and used in the following experiment: 



"Aug. i to 7, 1918. Under stones on the shore of Trunk 

 Island, Harrington Sound, 4 very pale A. atra secured; blue- 

 black pigmentation was apparent only at the very tips of the 

 siphons. These were kept in laboratory dishes lighted by a 

 northwest window, some little direct sunlight falling upon the 



