124 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



there are opposed tendencies apparently inlierent in the strain, 

 such as the assumption of a heavily plated tegmen by the Camerata, 

 the cause for which may be far to seek and which of necessity may 

 never definitely be determined. The modifications of the arms 

 proper wliich would be of value in an eleutherozoic existence are 

 on the other hand not incompatible with their functions as organs 

 of purely statazoic forms. 



The bracliial appendages of the Pelmatozoa subserve two vital 

 functions — the gathering of food and the bearing of sperm or ova. 

 Obviously, the greater the surface area exposed by the arms of a 

 crinoid, the more perfectly will the}^ fulfill their mission. In order 

 adequately to extend this surface area, a musculature of sufficient 

 strength and complexity to spread and close the rami is essential. 

 The arm area is increased cliiefly by bifurcation of the rami and by 

 pinnules. Increase of arm area, unless carried to excess, as in certain 

 Adinometra, is of positive value in locomotion, as offering a greater 

 surface for catching the water. The modifications of the muscula- 

 ture to adapt it the more perfectly to function as a motor system 

 would be simply an increase in the complexity of the system already 

 possessed. So it may be assumed that while the modifications of the 

 arms tend to be mutually strengthened, the tendencies in the modi- 

 fications of the vault and theca in general may w^ork at cross pur- 

 poses in forms having variable statozoic and eleutherozoic ancestry. 



Up to a certain limit, as before noted, an increase in the arm area 

 of a crinoid would prove of advantage in perfecting these appendages 

 as swimming organs. A similar result nught be even more effectively 

 brought about by the broadening of the rami themselves. Such 

 broadening is to be noted in the genus Eretmocrinus, the remarkable 

 paddle-shaped arms of wliich are so well known, A quite similar 

 broadening of the rami is to be found in Aorocrinus, Coelocrinus, and 

 Dorycrinus, although such structures have apparently not hitherto 

 been noted in the case of these genera. This remarkable broadening 

 of the rami may be explained on other grounds — as, for example, 

 giving greater food-catching area. The same result could be more 

 expeditiously and advantageously attained by a slight lengthening 

 of the pinnulse, or by bifurcation of the arms such as is common 

 among closely related genera. The beautiful adaptation of these 

 arms to swimming conditions is at once apparent, and it does not 

 seem highly improbable that such was an important part of their 

 functions. Again, we have the hooks on the pinnulse of Cactocrinus 

 (PL 5, figs. 4, 5) which hold the pinnulse together much after the 

 fashion in which the barbs of a feather are joined. So fastened 

 together, the pinnulse might w^ell have offered a most effective sur- 

 face for catching the water in swimming. 



