DISCOVERY OF THE DISK OF ONYCHOCRINUS 503 



the observed facts. Such shifting in the position of the rectum is a fact 

 well known in the embryology of the Echinoderms ;^ and it may be 

 remarked that there are instances, both among the Camerata and the 

 Inadunata, where the anus passed out through the test below the 

 level of the arm-bases. 



Fortunately for the purpose of our comparison, the only Crinoid 

 whose embryology we know belongs to the Flexibilia. It has been 

 the subject of elaborate and splendid researches by several distin- 

 guished and able investigators. The course of development of the 

 anal plate in the larva of Antedon, as brought out by the works of 

 these eminent men, lends force to the above suggestion. 



Sir Wyville Thomson's account^ of it is as follows: 



About the period of the development of the second radials, a forked spicule 

 makes its appearance in one of the interradial spaces between the upper portion 

 of two of the first brachial .plates. This gradually extends in the usual way 

 till it becomes developed into a round, cribriform, superficial plate. Simul- 

 taneously with the appearance of this "anal" plate, a caecal process like the 

 finger of a glove rises from one side of the stomach and curves toward the plate. 

 The plate increases in size, becomes inclosed in a little flattened tubercle of sar- 

 code, and maintaining its upright position it passes slightly outwards, leaving a 

 space on the edge of the disk between itself and the base of the oral plate imme- 

 diately within it. Toward this space the caecal intestinal process directs itself. 

 It rises up through it in the form of an elongated tubular closed papilla. The 

 summit of the papilla is finally absorbed, and a patent anal opening is formed. 



Dr. W. B. Carpenter continued the researches begun by Sir Wyville 

 upon the development of Antedon rosaceus, and he has given, ^ with 

 most admirable illustrations, the complete history of the anal plate. 

 For the convenience of readers who may not happen to have this 

 work at hand, I reproduce a few of his figures — some on a different 

 scale of enlargement — which will assist me in explaining the thought 

 I have in mind. In quoting some parts of Carpenter's description, 

 I refer to the figures on my own plate instead of to his original num- 

 bers. He gives the following statement of the first appearance and 

 subsequent history of this plate (p. 727): 



1 Bury on "Early Stages in the Development of Antedon rosacea," Philosophical 

 Transactions, 1888 B, p. 294. 



2 "On the Embryology of Antedon rosaceus," Philosophical Transactions, 1865, 

 P- 529- 



3 Philosophical Transactions, 1866, pp. 671 £f. 



