NO. 184G. ON CERTAIN ELEUTHEROZOIC PELMATOZOA—EIRE. 33 



Isocrinus and Metacrinus. — There is much evidence available 

 provmg conclusively that among certain recent crinoids provided 

 with a well-developed column, a free or semifree existence is largely 

 maintained. This conclusion was perhaps first reached by Sir 

 Wyville Thomson, and the evidence was subsequently verified and 

 added to by Carpenter and other writers. Carpenter's (1884, p. 18) 

 statement of the case as affecting the genus Isocrinus is here given. 

 "At the same time there appears to be ample evidence that a Penta- 

 crinite may lead the same sort of free life that a Comatula does, 

 attaching itself temporarily by its cutI." Sir Wyville Thomson long 

 ago pointed out, in the case of Pentacrinus decorus (1864, p. 7), "that 

 the animal seems to have had the power of detaching itself" at any 

 of the syzygies of the stem in the same sort of way as the arms are 

 thrown off during life or break up after death. He described an 

 individual in which the stem terminated below in a worn and rounded 

 nodal joint, and he supposed it "to have finally parted from its 

 attachment and to have led a free life." He stated some years 

 afterwards that this was the case in all the complete specimens which 

 he had seen, "showing that the animal must have been for long free 

 from any attachment to the ground." He then went on to describe 



I the same condition as it occurs in Isocrinus wyville-thomsoni. "All 

 the stems of mature examples of this species end mferiorly in a 



1 nodal joint surrounded by its whorl of cirri, which curve downwards 

 into a kind of grappling root. The lower surface of the terminal 

 joint is in all smoothed and rounded, evidently by absorption, show- 



I ing that the animal had for long been free. I have no doubt what- 

 ever that this character is constant in the present species, and that 

 the animal lives loosely rooted in the soft mud, and can change its 

 place at pleasure by swimming with its pinnated arms; that it is, 

 in fact, intermediate in this respect between the free genus Antedon 

 and the permanently fixed Crinoids." The recent species to which 

 a semifree existence has specifically been ascribed by Carpenter (1884, 

 p. 19) other than the ones already noted are: Isocrinus parrae (maclea^ 

 ranus), I. alternicirrus, and Metacrinus angulatus. 



The phenomena associated with the assumption of an eleutherozoic 

 habit by the recent forms are of very considerable interest, and are 

 of great value because of the perfect preservation of the material. 

 Being fairly certain as to the conditions under which such types have 

 acquired freedom, and knowing to a certain extent the habits of the 

 crinoids, one may draw certain general conclusions relative to the 

 fossil forms in regard to which we are not so well informed. 



Method of detachment among the Pentacrinidx. — The disruption of 

 the column immediately distad to a nodal, and the subsequent sec- 

 ondary deposition of stereom on the exposed face, as shown in 



94428°— Proc.N.M.vol.41—H-^^3 



