46 EMBEYOLOGY OF THE STARFISH. 



with the rapidly increasing deposit of limestone cells, it is soon hidden 

 from view, and I have not been fortunate enough to find it again in 

 more advanced young. I am therefore unable to say where the anus opens 

 outside, though it undoubtedly discharges, at this time, through one of the 

 many limestone cells. Owing to the difficulty of tracing its ojiening in 

 the dfedalus of round cells, I am not able to state this positively, never 

 having seen, from any point, discharges of fecal matters. Like the mad- 

 reporic body, it is not yet upon the abactinal area, but on the actinal 

 side, near the edge of the disk. The madreporic body itself would 

 have been lost in a similar manner, had it not been possible to track it 

 by means of its connection with the circular tube (PI. VI. Flrj. 12) ; and, 

 even then, it was only by the closest attention, and at moments when 

 the position of the young Starfish was especially favorable for the inspec- 

 tion, that the opening of the madreporic body could be distinguished 

 from that of the surrounding limestone cells. 



[With regard to the functions of the mouth of the Pluteus and its sub- 

 sequent fate in the young Starfish and Ophiuran, my observations as well as 

 those of Metschnikoff would show that it becomes the mouth in both. 

 This does not seem to be the case in Auricularia, and the fate of the 

 openings (both the anal and oral) of the Pluteus of Echinoderms is not 

 yet definitely known for all the orders. Additional observations are needed 

 on this point. Embryological studies on Mollusca would seem to favor 

 the formation of a new mouth distinct from that of the early stages of 

 the embryo, but the direct observations on Echinoderms all tend to prove 

 that there is no new opening formed, and that the mouth of the Pluteus 

 passes directly into that of the young Echinoderm. 



Selenka shows for Holothuria also that the original opening of the Plu- 

 teus becomes the permanent anus.] 



Formation of the Actinal Limestone Surface. — The actinal side of the disk 

 is at first a narrow flat band (PI. VI. Fig. 3), following the general out- 

 line of the rays. This band increases in breadth, loses its convex outline, 

 and soon reaches the terminal tentacle, when the actinal band has assumed 

 a pentagonal shape. Inside of this small pentagon is situated the ambu- 

 lacral system, entirely independent, as yet, from the limestone deposit on 

 the actinal surface, the whole rosette of tentacles expanding and contract- 

 ing, with perfect liberty, in every direction. This freedom soon ceases; 

 the points of the limestone pentagon develop ra23idly towards the centre 



