NO. 184G. ON CERTAIN ELEUTHEROZOIC PELMATOZOA—KIRK. 35 



the column would scarcely serve to separate the crinoid from its base 

 of attachment. Whether such disruption was voluntary or per- 

 formed more or less through the agency of outside forces, it is impos- 

 sible to state. Involuntary detachment in the majority of cases 

 seems the more reasonable hypothesis, however. 



Concerning the exact steps in the acquisition of a detached exist- 

 ence in the case of the Pentacrinidae we can not, of course, be abso- 

 lutely certain. The evidence at hand permits us to draw some 

 rather definite conclusions in regard to the matter, however. A 

 statement of the conditions under which it is thought freedom is 

 acquired, and of the consequent modifications in the structure of 

 the animal, may be of value as meeting some of the objections that 

 can be brought agamst the views of Thomson and Carpenter. It is 

 possible that in different cases we have to deal with somewhat dif- 

 ferent factors — or, rather, with factors operating in variable degrees. 

 On the wdiole, however, I think we are justified in assuming that the 

 process is essentially uniform in its action. 



Knowing as little as we do in regard to the embryology and very 

 young stages of the Pentacrinidse, we are scarcely able to form 

 definite conclusions in regard to the nature of the immature column. 

 The specimen of Isocrinus decorus (PI. 4, fig. 2) gives us some inter- 

 esting data, however. In this specimen it may be noted that the 

 column up to within a short distance of the theca is composed of 

 round ossicles. The proximal portion of the stem shows the charac- 

 teristic pentalobate cross-section that distinguishes the adults of this 

 type. It is probable that the distal portion of the column as shown 

 represents the actual shape of the ossicles in the young and does not 

 indicate a secondary deposition of stereom filling the reentrant 

 angles of the columnals. Were the change in form brought about 

 through such a process we would find a gradual transition instead 

 of an abrupt change, and some of the altered columnals would be 

 pentagonal in outline. Such, however, does not seem to be the case. 

 The comparatively rapid change from round to pentalobate ossicles, 

 such as may be noted in this specimen and even more clearly in a 

 specimen of the same species figured by Carpenter (1884, PI. 25, 

 fig. 2), perhaps indicates a resting stage in the development of 

 the organism after which the typical pentalobate columnals are 

 acquired. 



It is approximately at this period in the fives of the organisms that 

 detachment occurs. As stated above, this disruption is abrupt. In 

 all probability the point of separation comes well dowTi in the stem. 

 It is probable, however, that there is no definite point for the dis- 

 ruption of the column. Among these young specimens the length 

 of the stem is apparently quite variable, indicating such to be the case. 

 It is scarcely possible that in these individuals any considerable 



