16 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



in plate vii, figure 8, and in the normally fixed stages (plate vii, figures 3, 

 6, and 7) they have completely disappeared. This is coordinated with the 

 mixing up of the ectoderm with the mesenchyme (cf. p. 11-12), which 

 process has been completed by the time the larva has attached itself. Some 

 few cells may still remain attached to the surface by means of more or less 

 branching prolongations (plate vii, figures 6, 7, and 8). That these "multi- 

 polar" cells are the remnants of the ectoderm cells can hardly be doubted, 

 for they are not found in the Pentacrinoid stage. 



The suctorial disk loses its special histological character as soon as the 

 fixation has taken place. The same is the case with the larval nervous system. 



The exact time when the hydrocoel ring closes has not been ascertained, 

 but it takes place during the stage of transformation from free-s-nimniing 

 larva to young Pentacrinoid, and the important organs, the pore canal and 

 the stone canal, are formed also during this period. The pore canal was seen 

 first in the 4-day-old, free-swimming embryo, from \^'hich figures 5 and 6, 

 plate VI, are drawn. It still has no exterior opening. Its appearance in 

 plate VI, figure 6, as a small ring lying within a more spacious lumen must be 

 due to some contraction by the preservation. The stone canal (plate vi, 

 figure 8) is quite short; there are only two sections between its rising from the 

 hydrocoel ring and its opening into the parietal canal (plate vi, figure 9). 

 In the section represented (plate vi, figm'e 9) a thickening is seen in the left 

 oral mesentery, an ovoid mass of cells lying in the space between the epithe- 

 hum of the oral and aboral coelom and separating them from one another. 

 It seems certain that this corresponds to the primary genital gland found in 

 Antedon by Russo (op. cit., pp. 10-14), a rudimentary organ, which soon 

 disappears, but which, according to Russo, is of great morphological impor- 

 tance, being homologous with the genital gland of the Holothmians, while 

 the axial organ of the Crinoids is a thing apart, and not at all homologous 

 with the ovoid gland of other Echinoderms. I shall discuss these exceed- 

 ingly interesting and important morphological relations in the general part 

 of this memoir. 



Russo has found the first traces of this gland in Antedon in Pentacrinoids 

 which had already been attached for some 5 or 6 days and had aheady a 

 long stalk. In Tropionietra it appears somewhat earlier. I have been unable 

 to see this gland in decalcified and stained Pentacrinoids of this species, 

 while in some of the other species studied I have found it easily observable 

 in unsectioned Pentacrinoids (plate xii, figure 5, Compsometra serrata; plate 

 XXI, figure 6, Isometra vivipara). 



The formation of the primary tentacles needs no description. It may be 

 seen directly on the figures (for instance, plate vi, figure 1 ; plate vii, figure 7) ; 

 the explanation of the figures gives the necessary details. 



The coelomic vesicles have assumed their final position, the left at the oral 

 end, between the hydrocoel and the entoderm, the right at the aboral end 



