. Amphioxus and the ancestry of the vertebrates [microform]. Vertebrates; Chordata; Fishes; Ascidiacea; Vertébrés; Cordés; Poissons; Ascidiacés. BALANOGL OSSUS. 245 ^nor I the nged er of more ear a oxus. .s the ilk of it the racter 3d-like entary dermic irtially ve rise les the Verte- mouth, lis, this e noto- 5 many iss. It ;e-fibres )ver the ;nvelops enings. ng the s ii; the median longitudinal nerve-cords. In the region of the collar the dorsal nerve-cord becomes entirely separated from the ectoderm, and this portion of it contains, at least in young individuals, a central canal which, from
. Amphioxus and the ancestry of the vertebrates [microform]. Vertebrates; Chordata; Fishes; Ascidiacea; Vertébrés; Cordés; Poissons; Ascidiacés. BALANOGL OSSUS. 245 ^nor I the nged er of more ear a oxus. .s the ilk of it the racter 3d-like entary dermic irtially ve rise les the Verte- mouth, lis, this e noto- 5 many iss. It ;e-fibres )ver the ;nvelops enings. ng the s ii; the median longitudinal nerve-cords. In the region of the collar the dorsal nerve-cord becomes entirely separated from the ectoderm, and this portion of it contains, at least in young individuals, a central canal which, from its origin and relations, was shown by Bateson, and more recently by Morgan, to be homologous with the central canal of the vertebrate spinal cord. Anteriorly the dorsal nerve-cord becomes continuous with a specially dense tract of the general nerve-plexus at the inner posterior surface of the. Fig. 114. — Diagram of the org^inisation of Ralanoqlossiis, fiom the left side. (From a drawinij kindly lent by Professor T. H. MoRdAN.) ill. Alimentary canal, bc^. Coelom of proboscis (anterior or pr;Toral iiodv- cavity). be-. Coelom of collar, hc^. Coelom of trunk, Blood-vessel, proceed- ing from the so-called heart (which lies at base of proboscis above the noto- chord) to the ventral blood-vessel, ch. Xotochord. covt. Coinmissure, between dorsal and ventral nerve-cords, dn. Dorsal nerve-cord, separated from the integu- ment in the collar-region. Dorsal blood-vessel. f[l. Proboscis-gland; modified ccelomic epithelium surrounding heart and front end of notochord. m. Mouth, Pulsating vesicle, lying inside the " ; Ventral blood- vessel, Ventral nerve-cord. proboscis (Fig. 114). This proboscidian plexus thins out somewhat towards the anterior extremity, but nevertheless forms a complete nerve-sheath for the proboscis and indi- cates the sensitive character of the latter (Fig. 115). The ventral nerve-cord does not extend into the region of
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectfishes, bookyear1894
