LOWER PALAEOZOIC FAUNAS 133 



symmetrical animals. By adoption of sessile habits, 

 more or less perfect radial symmetry (usually pen- 

 tamerous) was acquired, and all but obliterated the early 

 bilateralism. Many forms, by return to a free mode of 

 life, superposed a fresh bilateral symmetry upon the 

 radial one, but rarely succeeded in masking it completely. 

 The primitive, bilateral stage is not known among 

 fossils, although some of the early Cystids approach it 

 very nearly. Probably the phylembryonic ancestors 

 were without hard-parts, or provided with loose, delicate 

 plates and spicules unsuited for fossilization. Certainly 

 they must have lived in pre-Cambrian times. The 

 sessile stage was brought to perfection in Palaeozoic 

 times, and is all but abandoned in the recent Echinoderm 

 fauna. Secondary freedom seems to have been attained 

 very early, for some of its possessors are known from 

 the Cambrian; but the quality is particularly char- 

 acteristic of Mesozoic and Cainozoic forms. Palae- 

 ontological evidence is thus in accord with inference 

 drawn from ontogeny and comparative morphology 

 the Pelmatozoa (sessile forms) are, in the main, older 

 than the Eleutherozoa. The quality is well demon- 

 strated by the proportions of attached and free classes 

 now living. Three groups of Eleutherozoa (Holo- 

 thurioidea, Stelleroidea and Echinoidea) are perhaps 

 more abundant in recent seas than as fossils, the 

 Edrioasteroidea (permanently or temporarily sessile 

 types usually classed with the Pelmatozoa) closed their 

 brief career in the Carboniferous period. But of the 

 three classes of Pelmatozoa (Cystidea, Blastoidea and 

 Crinoidea) Crinoids alone have survived the Palaeozoic 

 era, and an overwhelming majority of modern types are 

 eleutherozoic in habit, though not in structure. 



Cystidea and Crinoidea were the predominant Ech- 

 inoderms of the Lower Palaeozoic. The former class 



