274 W. J. Ceozier, 



CowLEs, 19Jlb, in Starfish and sea-urchins; Holmes, 1912, in Arbacia; 

 MacCuedt, 1912, in Asierias, Thyone, etc.). 



a)PhototaxisandtheShadowReflex. The concomittant 

 presence of negative orientation by light and a negative re- 

 action to shading in the behavior of a single animal is an important 

 point in the discussion of the tropism idea. No stronger proof of 

 the distinctness of the two categories of light reactions first made 

 by LoEB (LoEB, 1893) could possibly be desired. Further, Banceoft 

 (1913) has clearly demonstrated that in certain strains of Euglena 

 grown under controllable cultural conditions, the phenomena of 

 "differential sensibility" (Unterschiedsempfindlichkeit) and heliotropic 

 orientation, so long confused by workers of the Jennings-Mast 

 school, are things which may be subjected to experimental variation 

 independently. He believes that in all probability different photo- 

 chemical reactions are involved in the two modes of stimulation. 

 For Holothuria, as I shall attempt to show, a simpler hypothesis is 

 possible. 



b) Theory of Photic Sensitivity. 1) A pigment complex 

 consisting of at least two well differentiated substances is found in 

 the integument of H. surinamensis and captiva. One of these, the 

 more deeply situated of the two, is a brown material slightly so- 

 luble in strong alkalies. From alkaline solutions it is precipitated 

 by an excess of acids as a granular mass insoluble in alcohol, ether, 

 acetone or formalin. It is not extracted from the tissues by alcohol 

 or formalin. It would appear to be a melanin substance, and is 

 probably chemically related to the dark brown pigments which 

 occur almost universally throughout the genus ^) (cf. Beiot, 1906 j 

 Edwaeds, 1908 analysis of the tegumentary colors of H. floridana 

 and atra\ and taxonomic lists, as: Fishee, 1907). 



The other pigment is soluble in formalin, alcohol, acetone, and 

 distilled water. It is of a greenish-yellow hue, and is seen most 

 obviously in the tentacles, pedicels, ventral surface, and tips of the 

 papillae. This second substance arises in the embryo later than 

 the first, for Edwaeds (1909) describes the appearance of groups of 

 green pigment spots which later become evenly distributed over the 

 whole surface of the previously brown larva (£?. floridana). Extracts 

 of this greenish pigment from H. surinamensis and captiva were 



1) H. rathbimi, though usually very pale in tint, also shows brownish 

 colors. 



