. Elementary text-book of zoology. Zoology. FIG. 153.—Sagartia nicea (after Gosse). free pole pierced by an oral opening situated on a flat or conical prominence, the oral cone. The mouth is surrounded by one or more circles of tentacles, and leads into a simple cylindrical body cavity (Hydroidpolyps), or through an cesophageal tube into a compli- cated gastrovascular cavity (Anthozoa, fig. 153). The disappearance of the tentacles gives rise to the so-called polypoid form, which consists of a simple hollow tube fur- nished with a mouth. The Medusa type.—The free-swim- ming Medusa consists of a
. Elementary text-book of zoology. Zoology. FIG. 153.—Sagartia nicea (after Gosse). free pole pierced by an oral opening situated on a flat or conical prominence, the oral cone. The mouth is surrounded by one or more circles of tentacles, and leads into a simple cylindrical body cavity (Hydroidpolyps), or through an cesophageal tube into a compli- cated gastrovascular cavity (Anthozoa, fig. 153). The disappearance of the tentacles gives rise to the so-called polypoid form, which consists of a simple hollow tube fur- nished with a mouth. The Medusa type.—The free-swim- ming Medusa consists of a flattened disc or arched bell of gelatinous or cartilaginous consistence, from the under surface of which hangs a central stalk, the manubrium, bearing at its free end the mouth. This inanubrium is frequently prolonged in the neighbourhood of the mouth into numerous lobes and tentacles, while from the edge of the disc arise a varying number of thread-like tentacles. The central cavity of the body, into which the hollow manubrium leads, is called the gastric cavity, and from it peripheral pouches or radial canals, the so-called vessels, run to the edge of the disc, where, as a rule, they are con- nected by a circular vessel. The movements of the Me- dusa are effected by the alter- nate contraction and dilatation of the muscular under surface of the bell, of the subum- brella. Rudimentary Medusae, in which the manubrium and marginal tentacles are absent, are found. They are called Medusoids, and do not acquire individual independence, but remain attached to the body of the Medusa or Polyp from which they are budded. The Medusae and Polyps, in spite of the important differences between them, are but modifications of the same plan of structure. A Medusa may be compared to a free, flattened Polyp, possessing a large gastric cavity and a muscular and enlarged oral disc. The Ctenophor type has fundamentally the form of a sphere,. FIG. 154.—Medusa of the Podocoryne came* wi
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
