IX. Genetic Relations to other Echinoderms. 1915 403 



So far as the change in mode of life is concerned, their hypothesis 

 is no easier than mine. Professor MacBride has made a number 

 of ingenious suggestions, chiefly dependent on "the fundamental 

 distinction which obtains at the present day between the habits of 

 Eleutherozoa, which in the majority of cases are scavengers, devouring 

 dead animals and organic detritus lying on the bottom, and those of 

 Pelmatozoa, which to this day feed on Plankton captured by currents 

 produced by the cilia covering their tentacles" (1914, Embryology, 

 p. 564). Now that Dr. Gemmill has broken down the barrier of this 

 41 fundamental distinction" the hypotheses based on it lose much of 

 their probability. 



If we imagine an Edrioasteroid with loose attachment, liable to be 

 overturned by currents, just as we know that individuals of Stromato- 

 cystis were overturned, then all we have to suppose is that some of 

 the overturned individuals were able to survive the accident. This 

 they would be able to do if they had fairly well -developed podia, 

 such as are indicated by the anatomical evidence. Even without the 

 overturning, the podia of such genera as Edrioaster and Dinocystis 

 might have subserved locomotion ; for in them the ends of the 

 subvective grooves were brought into almost direct contact with the 

 sea-floor. Indeed, it is hard to see how locomotion could have been 

 avoided. 



As for the alleged directness of individual development (which, as 

 already pointed out, is not direct in some important particulars), this 

 may well be more apparent than real. If it be direct, we are thrown 

 back on ' idiosyncrasy ' and ' hemiplegia ' and misfits 1 somehow 

 displaced '. If, on the other hand, the stalk represents the last trace 

 of a former pelmatozoic stage, we have to imagine that the greater 

 part of that stage has been cut out. But what is more natural ? In 

 Ophiuroidea the fixed stage is omitted altogether. In Asteroidea it 

 is not omitted but condensed. The later stage, in which the mouth 

 is already directed downwards, has been pressed back so as to 

 eliminate the supposed stage during which it passed upwards. The 

 preservation of that stage in the ontogeny would have been a useless 

 waste of energy. 



Dr. Gemmill says that my view introduces u very serious onto- 

 genetic difficulties". I am unable to see that the main ontogenetic 

 difficulties are any better explained by the Gemmill-MacBride theory, 

 which in its turn seems to me to introduce a fresh set of phylogenetic 

 difficulties. 



