322 Dr. F. A. Bather — Notes on Yunnan Cystidea. 



(Bather, 1900. Treatise, p. 61, fig. xxv), Cryptocrinus {op. cit., p. 70, 

 fig. xxvii, 2, after Jaekel), and other genera. 



The gonopore, on the other hand, is a simple hole, as often as not 

 through the middle of a plate. It further differs from the hydropore 

 in heing frequently (perhaps normally) closed hy valves, and these, 

 as in the case of the periproct, give it a rectilinear — often pentagonal 

 — outline. It is so small itself, and its valves are so minute, that 

 the structure often cannot be made out, even if the pore itself be not 

 obscured by the processes of petrifaction. 



It seems probable that both hydropore and gonopore original] y 

 emerged between thecal plates, and that they were subsequently 

 surrounded by the plate-stereom in the same way as the podia of 

 echinoids have become surrounded by the ambulacral plates between 

 which they originally passed. The gonopore of the Cystidea, being 

 the outlet of a single gland, has remained a simple pore and has been 

 wholly occluded by a single plate. It is not so easy to see why the 

 hydropore should affect two plates. The curious bilateral folding of 

 the stone-canal in Asteroidea and the alleged occasional paired 

 hydropore of the embryo might suggest that here was a relic of 

 the bilaterally symmetrical Dipleurula. On the other hand, the 

 obliquity of the hydropore-passage in Edrioaster and elsewhere 

 shows how two plates may be involved without any suggestion 

 of duplicity. The essential difference between hydropore and 

 gonopore seems to be the conversion of the former into a filter 

 through the formation of numerous minute pores connected by 

 a ciliated channel on the exterior. This channel extended from the 

 original opening in both directions. For the present this must be 

 taken as a fact, but we have still to explain the direction followed 

 and its frequent regular curvature. In later forms these primitive 

 features were obscured by the excessive folding that produced the 

 characteristic madreporite. 



If the preceding statements be accepted, we are provided with 

 a touchstone enabling us in most cases to decide which of two 

 visible openings is the hydropore, which the gonopore, or, in the 

 case of only one visible opening, to say which of the two it is. For 

 example, in Cheirocrinus constrictus Bather (1913, Trans. 11.. Soc. 

 Edin., XLIX, ii, p. 445, fig. 51), the " extended pore " that crosses 

 " the curved suture" is probably the hydropore, and the "poriferous 

 elevation at the other end of the plate " is probably the gonopore ; 

 the converse interpretation was tentatively suggested in the memoir. 

 In the " Treatise on Zoology" some openings are perhaps lettered 

 wrongly: in Echinosphaera, p. 53, fig. xiv, and Sphaeronis, p. 72, 

 fig. xxxviii, the tri-valved opening marked M, and in Protocrinus, 

 p. 75, fig. xlv, 1, the pore through the plate marked M, should all be 

 the gonopore. The test will be applied to Megacystis later. 



In comparing Megacystis with other genera, Carpenter went on 

 the belief that there was no hydropore or gonopore at all in 

 Agelacrimis, the Caryocrinidae, and Malocystis, and no separate 

 gonopore in seven other genera that he mentions. Therefore he was 

 ready to admit similar conditions in Megacystis. Since then we have 

 learned that there is a hydropore in the Edrioasteroidea (S. R. 



