NOTES ON SOME PROBLEMS OF ADAPTATION. 



proportion to the available food material, and was able to cor- 

 relate this finding with the forms of Ciona in different natural 

 environments. A. atra from among piled stones was noticed 

 (before Fuch's paper was known to me) to have siphons very 

 short in comparison with those of the more robust specimens 

 from open situations. The view is, therefore, permissible that 

 relatively deficient metabolism is in this instance responsible for 

 subnormal pigment deposition. 



The experiments of Miss Johnson ('13) regarding the pigmenta- 

 tion of amphibian larvae showed the kind of materials ingested, 

 rather than the absolute amount of food, to be important for 

 pigment formation. In Ascidia we are not dealing with a 

 melanin, however, but with a quite different type of pigment. 

 Unfortunately, one is not able to decide as to whether the larvae 

 ultimately producing the pale A. atra located under littoral 

 stones came in the first place to assume such sites in an accidental 

 way, or were carried there and, because of some deficiency in 

 vigor, failed to escape, or because they endured too long (or not 

 long enough) in the larval phase and became attached at a time 

 of day unsuitable for the operation of photic control of their 

 movements. 



The larva of Ecteinascidia, as I shall elsewhere describe, is 

 normally liberated on a falling tide, and the duration of its free- 

 swimming phase, together with its modes of response, are so 

 adjusted to the tidal rhythm that in most cases a new individual 

 (in this latter species, resulting in a colony) is laid down at about 

 low-water level; the exact location of the site of attachment 

 depends in an interesting way upon developmental changes in the 

 phototropism of the larva. (Parallel changes have since been 

 described by Grave, 1920, in the larva of Amarouciiim; but the 

 ethologic correlations, fairly clear in Ecteinascidia, have been as 

 yet incompletely set forth for Amaroucium.} Should similar 

 phenomena appear in the comportment of the larva of A. atra, 

 some transforming individuals might become too photonegative, 

 or might settle down at night, in either event perhaps becoming 

 affixed in a dark cranny among stones. 



4. Definite proof that darkness as such has no control over 



