422 NATURAL SCIENCE. June, 



many, the fact that there were eight of them would be difficult to explain, 

 though, of course, it might be suggested that this was necessary to 

 enable the animal to determine the direction in which the sound 

 reached it ; but, even then, the limited powers of locomotion would 

 seem to render such a sense useless, and hence the evolution of the 

 organs could not be explained by Natural Selection, nor yet, since, the 

 animal has no vocal organs, by sexual selection. 



Temperature may, perhaps, affect them, but if so surely it 

 would affect all alike, and eight would not, therefore, be more useful 

 than one or two ; nor is their structure explicable on the hypothesis 

 of sensitiveness to temperature. 



It may be well, therefore,, to enquire what are the conditions 

 under which the animal lives, and what are the dangers to which 

 it is exposed. 



The danger from living enemies may be left out of account, for 

 the abundant supply of thread-cells and the very small percentage of 

 matter useful for food which they contain make them almost useless 

 as food for other animals, and, even if it were not so, they have no 

 means of escaping from any animal which might pursue them. 



These animals are pelagic, that is, they live in the open ocean, 

 near its surface. The tissues of the body are so delicate that it is 

 difficult to lift one from the water without tearing it to pieces. The 

 dangers of the storm-tossed surface of the ocean for such an animal 

 are, in part at least, obvious, but it is less obvious how they are 

 avoided, especially if the animals be devoid of judgment and even of 

 consciousness. The view I propose to offer is, that the tentaculocysts, 

 without giving rise to sensation, serve to automatically steer the 

 animal in such way as to keep it out of the way of its chief dangers 

 and, at the same time, in the region where its food most abounds. 

 How this is effected can only be made clear when we know some- 

 thing about the nature of the disturbance which we call a wave. 



It is not easy to know very much about a wave without also 

 knowing more about certain branches of higher mathematics than is 

 usual among zoologists. Parenthetically, I may, as a biologist, admit 

 that the Senior Wrangler who defined a biologist as " a man who 

 cannot solve a mathematical problem," came sufficiently near the 

 truth to make the epigram a distinctly unpleasant one ! 



Those biologists who are undismayed by formidable formulas, 

 and who have not as yet studied wave-movements of water from the 

 mathematical point of view, will find the greater part of the problem 

 treated mathematically by Lamb (i) and Basset (2) ; but I will 

 assume that the Senior Wrangler was right, and that the biologist is 

 a man of observation rather than of mathematical investigation, and 

 I will, tlierefore, appeal rather to his power of observing than to his 

 mathematical faculty. 



It is unfortunate that at times in our dredging excursions, when 

 the waves are largest and best fitted for study, those of us who lean 



