252 M. Edmond Perrier on (he XXIX. — 7^6 Origin of the Vertebrates. By Edmond Perrier*. Since the researches of Kowalevsky (1866) the Vertebrates have been successively derived by authors from indeterminate animals (Scolecida), which were considered to have given rise at the same time to the Tunicates (Hasckel, 1866) ; from the AnneHd Worms (Semper, Balfour, 1874; Dohrn, 1875; E. Perrier, 1881 ; Leydwich Minot, 1897) ; from the primary !Merostomata (Albert Gaudry, 1883) ; from Balanoglosms (Bateson, 1884) ; from the Nemertines (Hubrecht, 1887) ; from the Arachnids (Patten, 1891) ; from the Crustaceans (Gaskell, 1891) ; from an animal allied to the Appendicu-larid« (Brooks, 1893 _; Willey, 1894). Differences of opinion such as these evidently imply either that the principles of zoology are still badly defined, or else that they are too frequently lost sight of, or else again that sufficient attention has not been paid to determining the nature of the characters of Vertebrates, the explanation of â– which had to be demanded from ancestral forms. We hope to show in the present paper that a rigorous application of undisputed principles leads to a unique solution of the problem, and that this solution is in every respect satisfactory. The following essential characters are exhibited by every Vertebrate : — (1) The body is bilaterally symmetrical and metamerically segmented throughout its entire length ; (2) an important extent of its external or internal surfaces, especially those devoted to the respiratory function, is clothed with vibratile cilia ; (3) during tlie embryonic period, at the very least, the anterior region of the alimentary canal always communicates with the exterior by means of lateral clefts ; (4) the circu-latory apparatus is closed, and exhibits a heart situated below the alimentary canal ; (5) the secretory apparatus is consti-tuted by a system of ducts which are repeated, in the embryo, throughout the whole length of the body, and provide the genital apparatus with its excretory ducts ; (6) above the digestive tract there extends, in the embryo, a solid cord of cells, the notochord^ around which are formed the vertebra? of the adult animal; (7) above thenotochord lies the central nervous system, all on the same side of the alimentary canal, deprived of an cesophageal ring, and exhibiting a considerable bulk ; (8) with * Translated by £. E. Au?ten from the ' Comptes Rendus,' t. cxxvi. no. 21 (May 23. 18PS), pp. U79-1486.
BioStor
