MONOGRAPH OF THE EXISTING CRINOIDS. 431 



In the case of partial loss the wound sometimes simply heals over with no 

 restitution of the missing part. This occurs frequently in the terminal portion 

 of very long cirri, such as those of the species of Ptilometrinse. More commonly, 

 however, there forms in the course of a few days a small bud, apparently con- 

 sisting of ectoderm and mesenchj'me elements which can not be differentiated. 

 Later an epithelium of more or less cubical cells appears. Meanwhile, with the 

 usual cell proliferation and active cell division, the endothelium of the cirrus canal 

 and of the enveloping nerve sheath begins to regenerate, and there is formed, 

 projecting into the bud, a hollow finger-like cap over the end of the canal. Imme- 

 diately there appear the beginnings of the skeletal structures in the form of a 

 fine spicule which gradually enlarges into a fenestrated plate. 



As in the case of the arms, the chief stimulus for the growth of the regenerating 

 bud appears to come from the nerve, about the periphery of which rapid cell 

 formation takes place. 



While in old cirri the band of connective tissue which divides the cirrus vessel 

 into two halves appears more or less solid, in the regenerating buds it is clearly 

 composed of two bands, which form a sort of canal. Keichensperger considers it 

 probable that a nourishing fluid passes along this canal and out of its open end to 

 the actively proliferating cells. It may be a blood vessel, since it is connected with 

 the connective tissue of the axial organ, which certainly contains blood vessels. 



EMBRYOLOGY AND DEVELOPMENT. 



The embryology and development of four closely related species of comatu- 

 lids — Antedon lijida of the Atlantic coasts of France and England, Antedon medi- 

 terranea of the Mediterranean, Antedon adriatica of the Adriatic, and Antedon 

 moroccana of the northern coasts of Africa — have been studied by a number of 

 able investigators, and the chief features are now relatively well understood. 



The latest and most exhaustive work has been done upon Antedon adriatica, 

 and this, monographic in scope, includes a comparative account of the conclusions 

 reached on every point by all the previous authors. 



The major part of the investigations have been carried out on Antedon medi- 

 terranea, and upon the embryology and development of this type we have two 

 most excellent treatises, besides numerous scattered notes. 



Antedon moroccana has served as the basis for very detailed studies, but the 

 results have been somewhat obscured by the fact that it was not differentiated 

 from Antedon bifida and A. mediterranea, both of which were under consideration 

 at the same time. 



All of the pioneer work was based upon Antedon lifida, but since the middle 

 sixties this particularly interesting species has received no attention, thanks to 

 the greater availability of material of the Mediterranean forms. 



The first step in the study of the development of recent crinoids may be said 

 to have been taken by John Vaughan Thompson, who in 1827 published an elabo- 

 rate memoir upon a supposedly new small stalked crinoid which he had discovered 

 in the Cove of Cork, Ireland, and which he called Pentacrinus europwus. This 



