MKIM'S.K OF WOODS HULK RKdION. 21 



observed— namely, that cloudy or foggy days are frequently better times for collect 

 ing from the surface than bright, clear weather? If. however, such a nocturnal, or 

 negatively heliotropic, babit exist, we must seek the scat of response in differenl 

 organs. If we may allow that sensory bulbs arc present in Gonioiu mus and are visual, 

 we shall he confronted with a variation of the problem in Pennaria, which is wholly 

 devoid of such organs, and without sign of ocellar bodies. It should be noted in 

 this connection that experiments on Pennaria as to the effects of darkness were 

 entirely negative in results; at the same time no medusa known to me is more 

 apparently responsive to twilight conditions in its liberation from the hydroid, and 

 in the prompt discharge of its sexual products immediately after. I have elsewhere 

 pointed out that Pennaria shows certain aspects of degeneration, and among them 

 the visual organ may have been involved. If such has been the case, the process 

 must have been a gradual one, during which the visual function may have become 

 more or less generalized and distributed over the entire nervous organization, or to 

 generalized sensory cells similar to those of many other well-known animals, as the 

 earthworm, for instance. 



The brilliant coloration of many medusas is too well known to naturalists to need 

 particular emphasis, and to the general reader it will suffice to refer to the accom- 

 panying plates, from some of which a better idea may be obtained than would be 

 given by means of verbal description. Like several of the problems already raised, 

 that of color is noteworthy, if not indeed among the most difficult associated with 

 medusoid morphology, in connection with which it has usually been considered. As 

 will be seen from the following discussion, there is good reason to believe that the 

 most hopeful outlook for its solution lies along the line of physiology rather than 

 morphology. 



As already pointed out, many medusae are apparently devoid of visual organs. 

 and this fact alone would seem to preclude the usual explanation of coloration as 

 found among animals -possessing eyes of any marked acuteness. Again, it has been 

 pointed out that many medusae are of abyssal habit, where solar light is almost if 

 not wholly absent, and where in creatures with or without eyes color as a physical 

 feature must necessarily be of minimum value. Many naturalists have speculated 

 upon these phenomena, and various theories have been proposed by means of which 

 it was sought to bring them into some sort of harmony with our ordinary conceptions 

 of color as a factor in adaptation and natural selection. It has been suggested that 

 the absence of solar light at great depths is measurably compensated for in the pres- 

 ence of phosphorescence, a property known to be possessed by not a few abyssal 

 animals, and that this is adequate for the recognition of colors, or to render colors 

 variously protective. 



While these views are interesting and somewhat suggestive, they seem to me to 

 fall far short of affording even an approximation toward anything like a solution of 

 the simplest aspects of the problem involved. That phosphorescence may afford 

 some small measure of illumination when possessed by segregated groups of deep- 

 sea forms may be true, but not more so than in the case of surface and littoral 

 animals of similar properties. So far as I am aware, there has been little, if any, 

 disposition to interpret phosphorescence among the latter as serving any such function; 

 and while this alone may not disprove for it a function very different under the very 



