enlarging animal (for circulation and respiration)" (p. 47). "I have attempted to show," he writes further on, "that the majority of the Triploblastica ... are built upon a common plan, and that that plan is revealed by a careful examination of the anatomy of Coelenterata; that all the most important organ-systems of these Triploblastica are found in a rudimentary condition in the Coelenterata; and that all the Triploblastica referred to must be traced back to a diploblastic ancestor common to them and the Coelenterata" (p. 68). The main assumption was that the neural or blastoporal surface must be homologous throughout the Metazoa, though it was dorsal in the Chordata, ventral in the Annelida and Arthropoda. He derived the central nervous system of the Chordata from the circumoral ring of the common ancestor by means of the hypothesis that both the pre-blastoporal and the post-blastoporal parts of it disappeared.[451] The characteristic relation of the central nervous system to the blastopore in Annelida and Vertebrates had already been pointed out by Kowalevsky,[452] who had also sketched a theory of the common descent of these two phyla from an ancestral form in which the nervous system encircled the blastopore. In 1882, before the publication of Sedgwick's papers, A. Lang[453] had put forward the somewhat similar view that the stomach-diverticula of the Turbellaria, which he had found to be segmentally arranged in certain Triclads, were the morphological equivalents of the enterocoelic pouches of higher animals. This view, however, he soon gave up.[454] Sedgwick's views found a supporter in A. A. W. Hubrecht,[455] who utilised them in connection both with his speculations on the relation of Nemertines to Vertebrates, and with his exhaustive work on the early development of the Mammalia. He postulated as the far-back ancestor of Vertebrates, "an actinia-like, vermiform being, elongated in the direction of the mouth-slit" (p. 410, 1906), and derived the central nervous system from the circum-oral ring of this primitive form, the notochord from its stomodaeum, and the coelom from the peripheral parts of the gastric cavity (p. 169, 1909). [424] Gegenbaur, _Zeits. f. wiss. Zool._, v., 1853. [425] Remak, _loc. cit._, p. 183, pl. xii. [426] Lereboullet, _Ann. Sci. nat._ (4) xviii., pp. 118-9, 1862. [527] Lereboullet, in Remak, p. 183 f.n. [428] Kowalevsky, _Mem. Acad. Sci. St Petersbourg_ (Petrograd), (7), x. and xi., 1866 and 1867. [429] A. Agassiz, _Contrib. Nat. Hist. United States_, v., 1864. [430] _Mem. Acad. Sci. St Petersbourg_ (Petrograd), (7), xiv., 1869. [431] "Embryolog. Studien an Wuermern u. Arthropoden," _Mem. Acad. Sci. St Petersbourg_ (Petrograd), (7), xvi., 1870. [432] _Die Kalkschwaemme_, 3 vols., Berlin, 1872. General chapters translated in _Ann. Mag. Nat. Hist._ (4), xi., pp. 241-62, 421-30, 1873.