Elementary text-book of zoology, general Elementary text-book of zoology, general part and special part: protozoa to insecta elementarytextbo00clau Year: 1892 359 of which two cells may already be recognised as primitive generative cells. As soon as these make their appearance in the entoderm, the latter becomes folded in such a way that the ai-chenteron is divided into a median and two lateral cavities. The layer of cells lining the lateral cavities becomes the mesoderm, and the contained cavities the two lateral compartments of the bod}^ cavity, while that of the middle cavity
Elementary text-book of zoology, general Elementary text-book of zoology, general part and special part: protozoa to insecta elementarytextbo00clau Year: 1892 359 of which two cells may already be recognised as primitive generative cells. As soon as these make their appearance in the entoderm, the latter becomes folded in such a way that the ai-chenteron is divided into a median and two lateral cavities. The layer of cells lining the lateral cavities becomes the mesoderm, and the contained cavities the two lateral compartments of the bod}^ cavity, while that of the middle cavity gives rise to the wall of the mesenteron or alimentary canal. The permanent mouth is formed at the end opposite to that at which the blastopore, which is now closed, was situated. There is but one genus, Sagitta Slab., of which several species, , Sagitta hiinmctata Krohn, S. germanica Lkt. Pag, from the Euro- pean seas have been more accurately described. Order 2.—Acanthocepiiala.* Elongated round loorms with jjrotrusible 2'>rohoscis furnished loith hooks; without mouth and alimentary canal. The saccular, often transversely wiinkled body begins -with a proboscis, which is furnished with recurved hooks and can be retracted into a tube projecting into the body cavity (sheath of the proboscis) (fig. 289, R and Rs). The posterior end of this sheath is fas- tened to the body wall by a ligament, and by retractor muscles. The nervous system (fig. 289, G) is placed at the base of the proboscis, and consists of a simple ganglion formed of large cells. Nerves are given ofiT from the ganglion anteriorly to the proboscis, and through the lateral retractors (retinacula) to the body wall (fig. 289, R). The latter supply partly the mviscular system of the body, and partly the Rj genital apparatus, in which there are, princi- pally in the male animal, special nerve centres consisting of ganglionic enlargements. Sense organs are entirely wanting, as also are mouth, alimentary canal,
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