20 



ON THE EMBRYOLOGY OF ECHINODERMS. 



"» 



the plutean forms is so similar that they might easily, on first examination, be mistaken 

 for larvae of the same orders. 



All I wish to show from the figure of this Pluteus (fig. 31) is that in Ophiurans, 

 as well as in Echinoids and in Starfishes, the Echinoderm is developed on the water- 

 tubes, — one (the left) developing the actinal, the other (the right) developing the 

 abactinal region, — that the tentacular pentagon is open in the larval condition, and 

 is only closed by a process of unwinding, some time after the actinal and abactinal 

 surfaces have been formed, though this closing takes place in Ophiurans and Echi- 

 noids while the Echinoderm still retains parts of the framework. In Starfishes, on 



the contrary, the closing of the pentagons takes place after the resorption of th 



Brachiolaria. From the position of the water-tubes it is self-evident that the loop 



of the pentagon of tentacles (fig. 31, i) 



one plane ; they 



water-tube 6, and we find all the essential features of the tentacular water-tube (w) of 

 the Brachiolaria. (See Proc. Am. Acad., 1. c, fig. 12.) The tube w is not connected 

 with the tentacular water-tube w\ It would be interesting to observe whether such a 

 connection takes place, also the mode of formation of the first limestone particles on 

 the water-tube w, as this would facilitate the study of the development of the same 

 parts in Echinoids where the opacity of the larva prevents accurate observations. 

 Miiller has, in nearly all his figures of the Ophiuran plutean forms, represented the 

 water-tubes as made up of two distinct parts, having no connection. This probably 

 arose from the strong contraction of the water-tubes in certain parts. I have noticed in 

 the larva? which I observed a marked tendency in the anal portion of the water-tubes 

 to contract, and thus apparently to divide off from the remainder of the water-tubes ; 

 this was, however, but temporary, and the moment afterwards the water-tubes had 

 assumed again their fully expanded shape, as in fig. 31, w, w '. The young Ophiurans 

 (figs. 32, 33) are remarkable for the total absence of the plates of the disk ; they are 

 not even indicated by the presence of Y-shaped limestone rods. The only calcareous 

 deposits we have (y,y, fig. 32) are evidently parts of the first arm-joints, the dorsal 



(y, fig. 32) and side arm-shields (/, fig. 32) of that joint, which 



present of 



but a few rods, indicating their future position. The arms e' are the last to be 



sorbed ; in the stage of fig. 32 they retain their full length ; the other 



arms. e". e"' e" 



ery contracted condition, as they have been almost entirely resorbed by the 

 young Ophiuran, leaving nothing but the extremities ; the rods of two of the arms are 

 not entirely resorbed (fig. 32). 



This young Ophiuran, seen from the lower side, fig. 33, shows that the tentacular 

 pentagon has entirely closed. The ambulacral system is similar to the tentacles of 



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