GENERAL PART. 65 



from the hydroccel and then gains an outer opening, the hydropore. Later 

 on, about the time of the metamorphosis of the larva into the Pentacrinoid, 

 the hydrocoel, which has gradually assumed the shape of a horseshoe, the ends 

 of which afterwards join one another so as to form a hydrocrel ring, comes 

 again into connection with the parietal canal by means of the stone canal 

 arising from one end of the hydroccel. The stone canal arises before the 

 lumen of the hydrocoel fuses into a ring. 



The parietal canal has a distinct anterior prolongation towards the apical 

 end of the larva in Antedon, as well as in Compsometra, Isometra, and Noto- 

 crinus, but not in Tropiomeira. This is then again a feature of no general 

 value in the embryology of Comatulids. 



Russo 49 maintains that the parietal canal does not separate off from the 

 hydrocoel in Antedon. As I have said (page 13), this statement of Russo 

 can not be held as a proof against Seeliger's direct statement that it does 

 separate from the hydrocoel. The fact that Russo has not observed the pari- 

 etal canal separate off from the hydrocoel can not weigh against the careful 

 observations of Seeliger, with which my own observations on Tropiomeira, 

 Compsometra, Isometra, and Notocrinus are in perfect accord. There can 

 therefore hardly be any doubt that it is a general rule in Comatulids that the 

 parietal canal separates from the hydroccel. As previously stated (page 13), 

 figure 12, plate n, of Russo's memoir, to which he refers specially as proving 

 the connection between the hydroccel and the parietal canal, evidently shows 

 the opening of the stone canal into the parietal canal that is to say, the sec- 

 ondarily established connection between the two vesicles. 



Very interesting facts are connected with the hydropore. Russo main- 

 tains that in Antedon the original hydropore and pore canal disappear 3 to 4 

 days after the fixation of the larva. After 3 or 4 days more a definitive 

 hydropore and pore canal ("canale petroso secondario") develop, formed by 

 an ectodermal invagination. This organ of the Crinoid is thus an entirely 

 new formation, and, as Russo emphatically maintains, not homologous with 

 the madreporite and pore canal of other Echinoderms. 



Regarding this point, my own observations are as follows: In Tropiometra 

 no outer opening of the hydropore was found before the young Pentacrinoid 

 stage; whether it then disappears and is formed again in a later Pentacrinoid 

 stage remains uncertain for want of material of such later stages. In 

 Compsometra the hydropore is formed during the larval stage; it is probably 

 closed in the young Pentacrinoid, but it can not be ascertained beyond doubt 

 on the preparations in toto, and there is no material of these stages for section- 

 ing. In Isometra vivipara the hydropore is formed early in the larval stage 

 in some specimens, but not in all; however, the possibility can not be denied 

 that the hydropore would have developed in all of them during the larval 

 stage, there being perhaps only some variation in the time of its appearance 



49 A. Russo. Studii su gli Echinoderrni. Atti Accad. Gioenia, xn, 1902, pp. 47, 48. 



