NO. 1846. ON CERTAIN ELEUTHEROZOIG PELMATOZOA—EIRK. 125 



If we concede a general participation of the Crinoidea in an eleu- 

 therozoic existence, it will be well to bear the fact in mind, as 

 affecting the type of arm-branching, the modification of articulations, 

 and the general structure of the arms. We know, for instance, that in 

 at least two species of Mariacrinus there is little or no ground for the 

 supposition of an attached existence. ]\Iay we not then legitimately 

 wonder if the curious arm modifications of the Melocrinidae, the 

 atrophy of the outer and the hypertrophy and fusion of the inner 

 rami, is not perhaps due to the assumption of an eleutherozoic exist- 

 ence by the stock? We may not expect identical or even similar 

 modifications of arm-structure in different lines, for that is not the 

 Avay of crinoids. We may expect, and we do fuid, if we look for 

 them, modifications peculiar to the several lines, but all tending to 

 the solution of the same problem. So it is with other crinoid 

 structures. 



Effect of an eleutherozoic existence on the symmetry of the Pelma- 

 tozoa. — The assumption of a completely detached form of life b}^ cer- 

 tain Pelmatozoa gives one an excellent chance to study the effect of 

 free movement upon a type whose radial symmetry has been impressed 

 upon it by countless generations of statozoic ancestors. We have 

 been assured by A. H. Clark (1909, p. 685) that radial symmetry 

 does not depend upon a sessile ancestry. The octopus cited as an 

 example to prove this assertion scarcely tends to confirm Clark's 

 statement, however. The amount of modification resultant upon 

 the change from a fixed to a free existence depends entirely upon the 

 type of locomotion adopted. In the case of a crinoid, with its five 

 equal rays, the changes in symmetry are not of great magnitude and 

 probably were brought about but slowly. The exocyclic disks of 

 Uintacrinus and Actinometra are perhaps directly to be attributed 

 to a free-swimming life. As opposed to the for the most part 

 freely swimming Crinoidea, we have to consider the crawling 

 Eleutherocrimis and Zygocrinus. Here the changes, as pointed out 

 in the discussion of the genera, are far reacliing in their effect upon 

 the structure of the animal. The amount of modification seems to 

 vary directly as the amount of friction applied to the body of the 

 animal while in motion. The body propelled by its appendages 

 through a uniform medium, suffers little change, but given a drag 

 on the body, as exacted by movement along the bottom, and we have 

 results expressed in no uncertain terms. Naturally, to be effective, 

 movement must take place in a given direction, as far as the orienta- 

 tion of the animal is concerned. 



Effect of a detached mode of life on the distribution of the Pelmatozoa. — 

 The almost universal assumption of freedom by the Pelmatozoa can 

 not have other than a most marked effect upon the distribution of the 



