Text-book of comparative anatomy . cavity. This gastrulaformation takes place in such a way that the blastopore is a median ventrallongitudinal slit. This closes from back to front, until anteriorly, excentrically,unly a small aperture remains. An equatorial ciliated ring, the preoral ciliatedring, appears early in the gastrula larva. In the cell thickening which carries thisring of cilia a circular nerve is developed. At the aboral pole the ectoderm thickensto form the neural plate, which carries a tuft of cilia. Directly round the narrowedblastopore the ectoderm becomes invaginated like
Text-book of comparative anatomy . cavity. This gastrulaformation takes place in such a way that the blastopore is a median ventrallongitudinal slit. This closes from back to front, until anteriorly, excentrically,unly a small aperture remains. An equatorial ciliated ring, the preoral ciliatedring, appears early in the gastrula larva. In the cell thickening which carries thisring of cilia a circular nerve is developed. At the aboral pole the ectoderm thickensto form the neural plate, which carries a tuft of cilia. Directly round the narrowedblastopore the ectoderm becomes invaginated like a funnel, and forms the larvaloesophagus or the stornodseum, which gradually approaches the anterior ventraledge of the preoral ciliated ring. The archenteron elongates downwards and back-wards. Several smaller cells are severed by fission from the primitive or pole cells ofthe mesoderm, and these are spread out in the segmentation cavity (primary bodycavity) and form various larval organs, muscle fibres and the larval head. m FIG. 178.— A, li, C, Three stages of development of the larva (Trochoiihora) of Eupomatus,from the side, m, Polar cells of the mesoderm ; md, mid-gut; fh, segmentation cavity ; sp, neuralplate ; wk, preoral ciliated ring ; st, stomod%um ; wki, postoral ciliated ring ; n, larval head nephri-dium ; ot, otolith ; an, anus (after Hatschek). nephridia. The primary body cavity lengthens. That half of the body whichlies behind and below the preoral ciliated ring assumes a conical form. Thepoint of the cone is the posterior end. From the posterior end to the mouththe body becomes flattened. On the dorsal surface the posterior end of theintestine opens by means of a small invagination of the ectoderm, the proc-todseum with an anus ; this occurs not far from the posterior end of the opposite to this, at the neural area, which is surrounded by the preoralciliated ring, lies the neural plate, which represents a part at any rate of the rudi
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
