68 SMITHSONIAN INSTITUTION 



the skeleton. Such shifting in the position of the rectum is a fact well known 

 in the embryology of the Echinoderms, 1 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. A review of our knowl- 

 edge of this movement of the rectum in the Recent crinoids will be useful at 

 this point. 



Until recently, the only crinoid the larval stages of which we knew was 

 Antedon, which has been the subject of elaborate and splendid researches by 

 several distinguished 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 2 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 radial 

 plates. This gradually extends in the usual way till it becomes developed into a round, cribri- 

 form, superficial plate. Simultaneously 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 enclosed in a little flattened tubercle of sarcode, 

 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 immediately within it. Towards 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. 



And further, on p. 540 : 



Upon the appearance of the second and third radial joints, the perisome between and 

 somewhat above two of the first radials rises into a rounded papilla, towards which a caecal 

 process of the digestive cavity is directed. On the outer side of this papilla a branching spicule 

 appears which rapidly extends into a round plate. This, the anal plate, grows, and afterwards 

 thickens precisely on the model of the basal and oral plates. 



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

 the development of Antedon rosacens, and he has given, 3 with most admirable 

 illustrations, the complete history of the anal plate. For convenience I repro- 

 duce 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 numbers. He gives the following statement of the first appear- 

 ance and subsequent history of this plate (p. 727) : 



Between two of the radials, and on the same level with them, an unsymmetrical plate early 

 shows itself, the subsequent relation of which to the vent proves it to be an anal plate (PI. A, 



1 Bury, H., Early Stages in the Development of Antedon rosacea; Philosophical Trans. Roy. Soc. 

 London, Vol. 179 B, 1888, p. 294. 



2 On the Embryogeny of Antedon rosacens, Philosophical Trans. Roy. Soc. London, vol. 155, 1865, 

 p. 529. 



s Philosophical Transactions Roy. Soc. London, 1866, pp. 671 ff. 



