NO. 1846. ON CERTAIN ELEVTHEROZOIC PEL2IAT0Z0A~K1RK. 97 



requirements of an eleutherozoic existence. As a result of this high 

 degree of speciaHzation it is difficult, if not impossible, to pick out 

 any specific structure and use it as the basis of comparison with any 

 special group of crinoids in the hope of proving genetic relationship. 

 The structure of the tegmen; the exocyclic mouth; the thinness of 

 the plates of the cup; the large visceral cavity, made possible by the 

 fixation of pinnules and introduction of interbrachials ; the length of 

 the arms; the presence of syzygies; all these are modifications for 

 which a free-swimming existence is directly responsible. In order then 

 to arrive at any sort of a satisfactory conclusion in regard to the rela- 

 tionships of Uintacrinus we must consider the form stripped of its 

 special modifications. To be sure, \vith these gone we have little 

 left, but even that fact is of considerable importance. 



The exocyclic tegmen illustrates, I believe^ an accentuated bilateral 

 symmetry induced by a pelagic existence. The marked regu- 

 larity in the swimming movements of eleutherozoic types such 

 as to be noted in living Comatulse, unquestionabty tends to bring 

 about a bilateral symmetry on the part of the organisms.^ Among 

 other Echinoderms, notably the Eleutherozoa, the acquisition of a 

 secondary bilateral symmetry is too well known to warrant extended 

 comment. Nor can the flexible tegmen be taken as positive evi- 

 dence of relationship with any special group. This, too, is simply an 

 expression of a free-swimming existence — a pliant disk making for 

 greater mobility and consequent arm freedom. The arms themselves 

 again exliibit great specialization, but there is nothing in their struc- 

 ture which might not be independently acquired. The fix:ation of 

 pinnules, although characteristic particularl}^ of the Camerata, 

 nevertheless need not be restricted to that group. In Calamocrinus 

 and Actinometra, for instance, we find a partial incorporation of the 

 arms and pinnulse into the calyx, and also in Dadocrinus, as pointed 

 out by Bather. In Uintacrinus such incorporation was the logical 

 thing. The interbrachial system does not, I think, show marked 

 affinities with any group. Its great irregularity, if anything, rather 

 argues against a close relationship with the Camerata. 



1 Mr. A. H. Clark has proposed an interesting hj^jothesis to explain the exocyclic disk of the Comatnlae. 

 According to him, this type of tegmen is evolved as the resxilt of the feeding habits of the crinoids. Mr. 

 Clark says that those forms with exocyclic disks constantly keep their digestive apparatus filled with 

 bottom mud, from wliich they extract the nutritive material. Such feeding habits necessitate the pos- 

 session of an unusually long gut, as one finds in animals which feed in this manner. When one considers 

 the limited capacity of the body cavity it is obvious that such an increase in the length of the gut must 

 result in a coUing of that organ. Concomitant with such coiling Mr. Clark maintains that the orientation 

 of the disk changes. Such an explanation is equally applicable to Uintacrinus, Mr. Clark holds, though 

 here it is not bottom mud but surface plants as a food supply to which the animal must adapt itself. This 

 explanation seems a possible one in the case of Actinometra at least, and is an alternative explanation 

 constantly to be borne in mind. 



The feeding habits and food of endocyelie and exocyclic forms should be noted with care. In the case 

 of a ninnber of Actinometra japonica the included food of wliich I have examined, I found no evidence 

 of bottom sediment. The intestine content was composed almost wholly of comminuted animal matter. 



94428°— Proc.N.M .vol.41— 11 7 



