66 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



(see page 37). The main thing is that the hydropore is certainly formed 

 during the early larval stage, at least in some specimens (plate xv, figure 9). 

 In the fully formed larva, ready to leave the marsupium, the hj'dropore is 

 obliterated, and in the young Pentacrinoid the pore canal has no outer open- 

 ing (plate XIX, figures 1 to 4). At the time when the arms are beginning to 

 branch, a neiv hydropore is formed (plate xx, figure 3; plate xxi, figure 3). 

 In Notocrinus the hydropore is closed in the larval stage (plate xxiv, figure 

 9; plate xxvi, figure 4); in Floronietra it appears to be closed in the young 

 Pentacrinoid (plate xxvii, figure 7). 



While my observations thus (partly, at least) confirm those of Russo 

 regarding the obliteration of the hydropore and the formation of a new pore 

 at a later stage, they do not confirm his statement that the pore canal itself 

 also disappears and that a new pore canal is formed by an ectodermal invagi- 

 nation. The pore canal was always found to be distinct; it is only its outer 

 opening that disappears. Whether the new hydropore is really formed by an 

 ectodermal invagination seems perhaps a little doubtful. The figures given 

 by Russo (plate ii, figures 34 and 36) do not appear to be a sufficient proof 

 of this; they show only that the outer end of the canal has a thickened epi- 

 thelium. In view of the fact that the original pore canal persists in the 

 other Crinoids examined, and that Seeliger says nothing about its disappear- 

 ance in Antedon,^" it seems probable that Russo was mistaken in finding 

 it formed anew by an ectodermal invagination, and one feels little inclined 

 to regard his observations and figures as definite proof that the new hydro- 

 pore itself is formed in this way. My own material is not sufficiently well 

 preserved histologically to give a definite solution to a question so difficult 

 to settle. 



The facts hitherto brought to light seem to support the conclusion that 

 it is a rule in Comatulids that the original hydropore disappears soon after the 

 fixation of the larva, sometimes even before the fixation, and that then a new 

 hydropore is formed, perhaps by an ectodermal invagination. It does not 

 appear that this temporary obliteration of the pore is so important a fact 

 as to necessitate the conclusion that the madreporic pores (together with the 

 later developing additional pores and pore canals) of Crinoids are not homolo- 

 gous with those of the madreporic system of other Echinoderms. Also in 

 some Euryalids new madreporic systems develop, so that there is one in each 

 interradius; but there is certainly no reason to hold the morphological value 

 of these secondarily developed madreporites as quite different from that of 

 the primary one. 



Tropiometra appears to be exceptional in having no hydropore in the larval 

 stage; the material available is insufficient to ascertain whether this holds 

 good also for Notocrinus. 



'" It should, however, be mentioned that Bury (p. 278) states that "during the transition to the 'Cys- 

 tid' stage the walls of the parietal canal share in the general histolysia." 



