An introduction to the study of the comparative anatomy of animals . orpuscles isparticularly well marked in the latter species. Between ectoderm and endoderm is a very thin layer ofcolourless gelatinoid material, distinguishable as a transparentline in optical section. This is the jelly, or mesoglaea, veryscantily developed in Hydra, but attaining great thickness andforming the greater part of the bulk of the body in manyCoelenterates. It is not formed of cells as the ectoderm andendoderm are, but is formed as a secretion of (probably) bothof these layers. The general structure of the tentacl
An introduction to the study of the comparative anatomy of animals . orpuscles isparticularly well marked in the latter species. Between ectoderm and endoderm is a very thin layer ofcolourless gelatinoid material, distinguishable as a transparentline in optical section. This is the jelly, or mesoglaea, veryscantily developed in Hydra, but attaining great thickness andforming the greater part of the bulk of the body in manyCoelenterates. It is not formed of cells as the ectoderm andendoderm are, but is formed as a secretion of (probably) bothof these layers. The general structure of the tentacles is the same as that ofthe body. They are simply hollow processes of the latter,each containing a prolongation of the gastrovascular cavity,bounded by the two cellular layers ectoderm and endoderm,with the mesogloea between. When retracted a tentacle isthick and bluntly finger-shaped, its surface thrown into a Digitized by Microsoft® HYDRA 225 number of coarse transverse wrinkles. When fully extendedit is attenuated and filiform, the wrinkles disappear, and the. Fig. 48- A, a longitudinal section of Hydra viridis, semi-diagrammatic; m, mouth;bd, basal disc ; en, endoderm ; ec^ ectoderm; gv, gastrovascular mesogloea is represented by a black line. B, portion of a longitudinalsection through an extended tentacle, highly magnified, to show thebatteries of nematocysts; nig., mesoglcea; «, nucleus of the ectodermcell containing a battery ; en, endoderm; en, cnidocil. C, a nematocystof the large oval kind enclosed in a cnidoblast; the thread partly ^j a large oval nematocyst with the thread coiled up, enclosed in acnidoblast; en, cnidocil. C^, C^, stages in the development of a , an elongate oval nematocyst, showing the short relatively thick threadeverted. E, a small oval nematocyst with everted coiled up thread ; thecnidoblast is produced below into a long contractile process. surface is covered by a number of warty projections beset withp Digiti
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Keywords: ., bookcentury1900, bookdecade1900, bookpublisherlondo, bookyear1900
