638 



MR. H. G. NEWTII ON THE 



begin to be approximated (PI. II. fig. 11). What may be the 

 mechanics of this migration of the bases of the tentacles froBi 

 the ring to the radial canals I can only snrmise. I conceive it 

 to consist essentially in the conversion of those areas of the wall 

 of the ring which immediately surronnd the bases of the radial 

 canals into the walls of the canals themselves, which are growing 

 rapidly at this time. 



While a full discussion of the significance of these arrange- 

 ments cannot be attempted in this place, it may be well to call 

 attention to the fact that the specalations of Semon as to the 

 phylogeny of Holothurians were in great part founded upon the 

 assumption that the alternation of the tentacle lobes of the 

 hydrocoel with the radial canals, observed in Synapta, was a 

 constant feature of Holotharian ontogeny. This assumption was 

 controverted by Ludwig, who, after examining an eight-day larva 

 of C. planci as his earliest stage, had the assurance to assert that 

 the tentacles arose from the radial canals and not from the ring. 

 A careful perusal of the same author's second paper, in which 

 earlier stages are described, makes it appear doubtful whether 

 the first appearance of the tentacles was ever observed by him at 

 all. The question therefore still remains to be solved : Which is 

 secondar}^ the alternation of the tentacles (as lobes of the 

 hydrocoel) with the radial canals, or the adult condition in which 

 they are outgrowths of the radial canals ? 



Diagvam sliowiiig tlie relation of tLe various derivatives of tlie hydrocoel iss 

 (A) Holotlmria floridania and (BJ Cucumcn-ia. 



Fer explanation of lettering see p. 640. 



A comparison of the arrangement of the primary tentacles in 

 Holothiiria Jloridana, according to the account of Edwards (4), 

 with that in Cucumaria perhaps throws some light on this matter 

 (text-fig. 1). In the development of that animal, as in Cucumaria, 

 there is an asymmetiical disposition of the tentacles in relation 



