PHYLUM VIII. 

 CHORDATA. 



Animals possessing the following structural features: (1) a noto- 

 chord, a cylindrical rod lying along the mid-dorsal line of the digestive 

 tube, from the dorsal wall of which it has arisen, and ventral to the 

 central nervous system; (2) a pharyngeal respiratory apparatus consist- 

 ing of paired gill slits in the wall of the pharynx, which place that struc- 

 ture in communication with the exterior; and (3) a tubular, dorsal central 

 nervous system. In the air-breathing chordates, the notochord and the 

 gill slits are present only in the embryo, and in the adult are replaced by 

 the spinal column and the lungs. 



The phylum contains about 37,000 species, of which over 35,000 are 

 vertebrates, grouped in 4 subphyla. 



History. This phylum was formed in 1874 by Haeckel as a result 

 of the epoch-making studies of Kowalevsky of Balanoglossus, ascidians, 

 and Amphioxus in the preceding decade. 



Key to the subphyla of Chordata: 

 Oj Body (in the American species) long and worm-like. 

 a, Body not worm-like. L ENTEEOPNEUSTA (Hemichordata) 



&! Body more or less sac-shaped 2. TUNICATA ( Urochordata) 



6, Body lanceolate 3. LEPTOCAEDIA (Cephalochordata) 



6 a Vertebrates (not included in this book) 4. VEBTEBBATA 



SUBPHYLUM 1. ENTEEOPNEUSTA.* (HEMICHOKDATA.) 



Chordata in which the notochord consists of a hollow, dorsal projec- 

 tion of the anterior portion of the digestive tract (Fig. 996,13). The 

 body is unsegmented and soft in texture, and is composed of three por- 

 tions, the preoral lobe or proboscis (1), the collar (2), and the trunk (3). 

 The subphylum is composed of two orders, the Balanoglossida and the 

 Cephalodiscida, the latter order containing a very few species of marine 

 and largely deep-sea animals which are not found near our coasts. 



* See "Les'Procordes," by Delage et He>ouard, Traite de Zool. Coner, Vol. 8, 

 1898. "The Development of Balanoglossus," by W. Bateson, Quart. Jour. Mic. Scl., 

 Vols. 24-20, 1884-1886. "Growth and Development of Balanoglossus," by T. H. 

 Morgan, Jour. Morph., Vol. 5, p. 407, 1891. "Die Enteropneusten des Golfes von 

 Neapel," etc., by J. W. Spengel, Fauna and Flora, etc., 1893. "Die Benennung d. 

 Enteropneustengattungen," by same, Zool. Jabrb., System. Ab., Vol. 16, p. 219, 1901. 

 'Hemichordata," Camb. Nat. Hist., 1904. 



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