ECHINODERMA GENERAL DESCRIPTION 33 



as far as possible. Since the factor determining the lines of 

 evolution appears to have been position with regard to the sea- 

 floor, this must no longer be contemned as " mere difference of 

 habit." The first Echinoderms were not necessarily fixed, but 

 fixation probably affected all representatives of the Phylum at an 

 early period and produced gradual changes, the first being the migra- 

 tion of the mouth and left hydrocoel to the upper pole. Those 

 forms in which the oral pole remained uppermost, whether actual 

 fixation by the aboral pole persisted or no, are to be distinguished 

 from those in which the oral pole again shifted, accompanied by loss 

 of fixation. Leuckart's term Pelmatozoa (1848), though primarily 

 connoting the actual or potential possession of a stalk, has come 

 into general use for the former group. The term Statozoa, pro- 

 posed by Bell (1891), implies absence of locomotion, and is 

 therefore not so great an improvement as to compel its adoption. 

 The term Crinoidea was extended by Roemer (1851) to include 

 all Pelmatozoa, but such extension does violence to the intentions 

 of J. S. Miller, the coiner of the name (1821). The forms with 

 oral pole uppermost may, it is true, be divided into classes ; but, 

 as maintained by Huxley and Ray Lankester, their genetic con- 

 nection is so evident that it should be recognised by the establish- 

 ment of a Sub-phylum, to which we shall continue to apply the 

 name Pelmatozoa. The included classes, as hereafter explained, 

 are the Cystidea, Blastoidea, Crinoidea, and Edrioasteroidea. The 

 remaining classes of Echinoderma have been placed together by 

 P. H. Carpenter and others as Echinozoa, but may more con- 

 veniently be spoken of as Eleutherozoa (a term originally used by 

 Bell in a sense excluding Holothurians). Their genetic connec- 

 tion, however, is only that due to descent from the Pelmatozoa ; 

 even if all Eleutherozoa descended from one class of Pelmatozoa, 

 they did so at widely differing periods. The Holothurians must 

 have cast loose before the genital organs had been affected by radial 

 symmetry, and are thus, as well as by the horizontal position of 

 the oro-anal axis and the retention of the M plane as sagittal, 

 sharply distinguished from Echinoidea and Stelleroidea. The two 

 classes last mentioned were with some reason opposed by 

 Leuckart to Pelmatozoa and Holothurioidea (or Scytodermata, 

 as he called the latter) as Actinozoa ; but they differ in important 

 features. If Cuenot's interpretation of the lacunar systems be 

 correct, it seems as though the Echinoidea branched off before 

 radial symmetry had greatly affected the coelomic lacunar system 

 derived from the axial sinus ; similarly the digestive system 

 retained its coiled and non-radiate arrangement ; moreover, the 

 sinking of the ambulacral water-vessels and nerves below the test 

 here diverged further from the Pelmatozoic type than is the case 

 in Asteroidea. The extension of the ambulacra nearly to the 



