110 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



It is probable that in both this and in the preceding forms locomo- 

 tion was effected by the animals crawling on the bottom. The, 

 appendages seem scarcely to be of sufficient size and strength to 

 warrant the conclusion that they could function as efficient swimming 

 organs unless aided by a considerable buoyancy of the theca, which 

 in these cases seems to be wanting. 



Eleutherocrinus and Zygocrinus differ from most other eleutherozoic 

 Pelmatozoa either in not being essentially free-swimming organisms 

 or m not crawling along the bottom with the theca raised above the 

 surface. In their manner of life they simulate the Echinoidea 

 among the Eleutherozoa and the Anomalocystidse among the 

 Pelmatozoa. A free-swimming existence was denied them, indeed, 

 by the lack of proper organs of propulsion. Their slender, com- 

 paratively weak pinnules, ranged along the ambulacral areas, were 

 incapable of the sweeping oar-Uke movements of the long stout 

 crinoid arms. They could do creditable service, however, in pushing 

 the crinoid along the bottom, 



Eleutherocrinus. — Eleutherocrinus (PL 10, figs. 6-8) was descended 

 from an essentially Pentremitidea-likQ ancestor. When the blastoid 

 became detached from its column, it naturally lay lengthwise. Two 

 points determined the side which was to lay next the bottom. In 

 the first place the anal opening had to be on the lower side to prevent 

 the excrement from fouling the spiracles and ambulacra. In the 

 second place, as many of the ambulacra were to retain their unim- 

 paired efficiency as possible. There are three possible positions 

 meeting the first requirement; upon the right posterior ambulacrum, 

 the left posterior ambulacrum, and upon the posterior interradius. 

 In order to meet the second requirement, however, the posterior 

 interradius must be eliminated from consideration. With the 

 blastoid resting upon it, two ambulacra would have been rendered 

 practically useless. Of the two positions left, the one with the left 

 posterior ambulacrum resting on the bottom was assumed. 



In motion the blastoid — not Eleutherocrinus as yet — progressed 

 backward, as it were. That is, the apical portion formed the forward 

 end. The reasons for this are obvious. If motion were to take 

 place in the opposite direction, the sediment of the bottom would be 

 pushed up over the oral openings. Moreover, the efficiency of tlie 

 pinnules would probably be lessened. 



The modifications in the structure of the blastoid as a result of 

 the assumption of this type of existence are marked, and offer a 

 beautiful instance of adaptation. The aboral end of the theca became 

 sharply pointed, rounding above and flattened below. The left 

 posterior ambulacrum became greatly shortened and widened, 

 coming to lie entirely in the plane of the oral face. Thus we have the 



