74 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



It proved that (in the specimen figured in plate ix, figure 2, Report on the 

 Echinoderms of Northeast Greenland) the said radial and the anal had both 

 been formed, being of the same size, the radial lying in the radial midline 

 and the anal lying to the left of it, between the oral and basal plates, a 

 partial resorption of the border of these plates having already taken place. 

 Apparently, a most remarkable difference from what obtains in the other 

 Comatulids, so far as hitherto known, was thus found to occur in this form. 

 Fortunately, however, there was another slightly younger specimen, which 

 proves that there is no such difference. In this specimen the anal plate was 

 found reaching from the midradial line somewhat in between the oral and 

 basal to the left of it — the resorption of the edges having already begun ; the 

 radial lies to the right of the radial midline, as in the other Comatulids, and 

 is slightly smaller than the anal. There can accordingly be no doubt that 

 the development of these two plates in Hathrometra prolixa takes place in 

 the same way as in the other Comatulids thus far known, and it appears to 

 be a general rule among Comatulids that the anal plate develops in the radial 

 midline, like the true radials, atid before any of those, the right posterior radial 

 developing last of all, to the right of the anal plate, outside the radial midline, 

 between the oral and basal, and only later on in the course of growth assximing 

 the radial position, pushing the anal plate out from this place towards the left, 

 against the right lower corner of the posterior oral. 



It is evident that these observations do not give the slightest support to 

 Dr. Clark's view that the anal of the Comatulids represents the radianal; 

 combined with the objections to this assumption raised by Dr. Bather,''= they 

 maj^ well be said to prove definitely that the anal plate of Comatidids is not 

 the radianal, which always originates below the right posterior radial, repre- 

 senting, in fact, the lower half of this plate. Do the observations on the 

 development of the anal plate then support the view that it is homologous 

 with the anal X? 



As has been sufficiently proved by Dr. Bather "the anal X is intimately 

 connected with the right posterior radius and is, in fact, a radial element 

 of the Crinoid skeleton. Accordingly we should not be surprised in finding 

 it in a radial position in the embryos, and no objection to its homologj^ with 

 the anal X can then be raised from the fact that it does develop in a 

 radial position. 



Dr. Bather has suggested that the anal X originated as a plate homol- 

 ogous with a brachial, and moved downwards and sidewaj's into the inter- 

 radius as the anal structures widened. The fact that this plate is the first 

 to appear and from the first has perfectly the appearance of being one of 

 the radials, does not seem to me to point towards this supposed homology 

 with a brachial, which further necessitates the supposition of a downward 



'•■ F. A. Bather. W.ichsmuth and Springer's Monograph on Crinoids. Fifth Notice (The Anal Plates). 

 Geol. Magaz., Dee. iv, vol. vi, 1899. 



