MOVEMENT 



All three groups are very rich in forms, and far surpass 

 all the other stems of the animal world in the perfection 

 of their locomotive apparatus. However, the disposition 

 and development of the skeleton as a passive support, 

 and the correlation of the muscles to it as active puUing- 

 organs, differ very much in the three classes, and are_ 

 the chief factors in determining their characteristic 

 types ; they show clearly (even apart from other radical 

 differences) that the three stems have arisen indepen- 

 dently of each other from three different roots in the 

 vermalia - stem. In the echinoderms the calcareous 

 skeleton is formed from chalky deposits in the corium, 

 in the articulates from chitine secretions of the epidermis, 

 and in the vertebrates from cartilage of an internal 

 chord-sheath (c/. Anthropogeny, chapter xxvi.). 



The remarkable stem of the sea-dwelling echinoderms 

 or "prickly skins" is distinguished from all the other 

 animal groups by a number of striking pecu iarities ; 

 prominent among these are the special formation of 

 their active and passive motor organs and the curious 

 form of their individual development. In this onto- 

 genesis two totally different forms appear successively 

 — the simple astrolarva and the elaborately organized 

 and sexually mature astrozoon. The small, free-swim- 

 ming astrolarva has the general structural features of 

 the rotatoria, and so shows, in accordance with the 

 biogenetic law, that the original stem-form of the 

 echinoderms (the amphoridea) belonged to this group 

 of the vermalia. I have briefly explained these struct- 

 ures in the History of Creation (chapter xxii.), and more 

 fully in my essay on the amphoridea and cystoidea 

 (1896). The little astrolarva has no muscles, and no 

 water-vessels or blood-vessels. It moves by means of 

 vibratory lashes or bands, which are attached to special 

 armlike processes at the surface. These arms are 

 regularly developed to the right and left of the bilateral 



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