MOVEMENT 



the echinoderms have the most advanced and com- 

 pHcated motor organs of all animals. Their historical 

 development is perfectly understood from its earliest 

 stages, since Richard Semon found, in his ingenious 

 pentact aeatheory (1888), the correct phylogenetic mean- 

 ing of the curious embryology of the echinoderms dis- 

 covered in 1845 by Johannes Mtiller. I endeavored in 

 1896 to establish it in detail, in relation to paleontological 

 discoveries, in the essay I have mentioned. 



The large stem of the articulata (the richest in forms 

 of all the animal stems) comprises three chief classes — 

 the annelids, Crustacea, and tracheata. All three groups 

 agree in the essential features of their organization, 

 especially in the external articulation or metamerism of 

 the long bilateral body, and also in the repetition of the 

 internal organs in each joint or segment. In each joint 

 there is originally a knot of the ventral nervous system 

 (the ventral marrow), a chamber of the dorsal heart, 

 a chi tine-ring of the cutaneous skeleton, and a corre- 

 sponding group of muscles. 



Of the three great classes of the articulates the annelids 

 are developed directly from the vermalia, of which both 

 the nematoda and nemertinae approach very closely to 

 them. The two other and more highly organized classes, 

 the Crustacea and tracheata, are younger groups, inde- 

 pendently evolved from two different stems of the 

 annelids. The annelids, or "ringed-worms" (to which, 

 e.g., the rain-worms belong), have mostly a very homo- 

 geneous articulation ; their segments or metamera repeat 

 the same structure to a great extent, especially the 

 subdermal muscles. In a transverse section we see in 

 every joint underneath the layer of concentric muscles 

 a pair of dorsal and a pair of ventral muscles. Their 

 epidermis has secreted a thin covering of chitine, in the 

 tubular worms a leather-like or calcified tube. There 

 are no bones in the oldest annelids; in the younger 



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