138 BULLETIN OF THE 



its resemblance to the tentacular loops of Brachiolaria, and from its 

 position ou the water-tube connecting with the water pore, I have no 

 hesitation in considering to be the first tentacular loop formed." 

 Balfour considers this structure an invagination of the external surface 

 of the larva, an infolding which later is to form the ventral region of 

 the Echinoderm, Metschnikoff * ascribes to A. Agassiz the discovery of 

 an invagination of the outer skin of the pluteus to form the body of the 

 future echinus. I have not been able to find in A. Agassiz's works, 

 quoted by Metschnikoff, that he has made such a " discovery," and cer- 

 tainly he does not give to the lettering of his figures the same interpre- 

 tation which Balfour does, when he says that the structure in question, t, 

 is a tentacular loop. 



PI. VII. fig. 3, represents the young of Echinarachnius formed on a 

 pluteus of the same general form as that figured in PI. VII. fig. 2. The 

 left water-tube has here formed the " rcsette " of five radial tubes, 

 which are seen in profile in the figure. The whole body of the pluteus 

 is not represented, but a portion of the edge of the stomach of the plu- 

 teus is seen on the right-hand side of the figure. The figui'e is a 

 representation from the dorsal side. 



The five radial bodies of the " rosette," one of which is lettered, wt, 

 are the water-tubes or the ambulacral divisions which, when seen from 

 one side, would form a five-rayed water system derived from the left 

 water vesicle. The system communicates with a tube which passes 

 through the mesoderm ic layer of the plutean body, and opens externally 

 by an opening, ma, which is supposed to be the madreporic opening. 

 This opening is at first situated near the base or proximal end of the 

 posterior rods. Its communication with the water-tubes has a retort 

 form, especially in older stages. The retort-shaped vesicle was observed 

 in a stage a little older, PI. VII. fig. 4, than the last to be in direct 

 commimication with the rosette-shaped system of five ambulacral ves- 

 sels. Each of the five radiating ambulacral tubes which form the 

 rosette extends outward from a central region, which is in direct com- 

 munication with the retort-shaped body. These radial ambulacral tubes 

 are at first simple, without lateral branches, coeca-like folds or loops, f 

 with transparent walls, the outlines of which can be plainly seen 



« 



* Op. cit. p. 41. 



t These five loops are supposed to be homologous with the " odd ocular tenta- 

 cles " of Arbacia and Strongrjiocentrotus. See A. Agassiz, Report on the " Chal- 

 lenger " Echinoidea, p. 8. They are supposed to be the same as the " Fiihler," F. 

 in Asterina. (See Ludwig, op. cit., PL VII. figs. 96, 97.) 



