rOEMATION OF THE AVATER-TUBES. 15 



as it increases in size, grows triangular, with rounded corners ; the de- 

 pression in which it is placed divides the larva into two very distinct 

 regions (PI. II. Figs. 19, 23, 25). Since the formation of the mouth, and 

 the change of position of the first-formed opening to an eccentric one, we 

 find the mouth and anus placed on one side of the larva. These open- 

 ings present, at this stage (PI. IT. Fig. 17), the same relations as the mouth 

 and anus of Clypeaster and Scutella-like Echinoids, while at a much earlier 

 period they are more like Pygorhynchus. 



If we now return to the water-system, we find that the two diverticula 

 («', w), mentioned above (PI. II. Figs. 15, 16), have entirely separated 

 from the digestive cavity (PI. II. Fig. 18), and are now distinct cavities, 

 having no connection whatever either with the cavity from which they 

 originated or with one another ; one of these cavities is entirely closed 

 {iv), the other («/) connects with the surrounding mediiun by means of 

 a very small opening, the dorsal pore {b, PI. II. Fig. 23, and isolated. 

 Fig. 17). Such is the appearance of an embryo at the close of the second 

 day after fecundation. 



Miiller never knew the origin of the water-tubes ; in his last paper 

 only he becomes aware that they are independent at first, but subse- 

 quently unite. It must he remembered, in reading his earlier papers, that 

 he sets at rest, in his last memoir, the doubts he expressed concerning 

 the independence of the two branches of the water-tubes ; in fact, to 

 obtain a clear conception of Miiller's views, it is advisable to read his last 

 memoirs first, to be able to adopt at once the corrections he himself 

 makes during the laborious course of his investigations. The problem- 

 atic bodies, however, still remained a puzzle to him, even at the time of 

 his last memoirs, as he was never aware that they were simple diver- 

 ticula of the digestive cavity, and were finally transformed into the two 

 independent branches of the water-tubes, uniting, in subsequent stages 

 of growth, to form the Y-shaped water-system. Van Beneden saw, in the 

 young Bipinnaria (Brachina Van Ben.), that the water-tubes are at first 

 separate, but he did not trace their mode of formation, and no other 

 observer has since returned to this subject. 



[Metschnikofi" states that in some cases there is but a single water-tube, 

 and that I have mistaken an accumulation of cells for a second water- 

 tube. I can only state that I have frequently repeated my observations 

 on the Pluteus of Starfishes, Ophiurans, and Echini, and have invariably found 



