found in hundreds, often in thousands, on every individual five-rayed astrozoon, we might say that the echinoderms have the most advanced and complicated motor organs of all animals. Their historical development is perfectly understood from its earliest stages, since Richard Semon found, in his ingenious pentact æatheory (1888), the correct phylogenetic meaning of the curious embryology of the echinoderms discovered in 1845 by Johannes Müller. 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 chitine-ring of the cutaneous skeleton, and a corresponding 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 nemertinæ approach very closely to them. The two other and more highly organized classes, the crustacea and tracheata, are younger groups, independently 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 homogeneous 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 bristle-worms (_polychæta_) one or two pairs of short unjointed feet (_parapodia_) are found in every joint.