. Elementary text-book of zoology. Zoology. 244 CCELENTEBATA. D nophora physiologically as an organism and its appendages a? organs. In this connection we may mention that the sexual medu- soid generation is so little independent that it only exceptionally (Velellidce) reaches the morphological grade of the free-swimming Medusa. In place of the attached and ramified hydroid-stocks we find in the Siphonophora a free-swimming con- tractile unbranched stem (hydrosoma),. which is rarely provided with simple lateral branches. The upper end of the hydro- soma is frequently dilated to the form of a f
. Elementary text-book of zoology. Zoology. 244 CCELENTEBATA. D nophora physiologically as an organism and its appendages a? organs. In this connection we may mention that the sexual medu- soid generation is so little independent that it only exceptionally (Velellidce) reaches the morphological grade of the free-swimming Medusa. In place of the attached and ramified hydroid-stocks we find in the Siphonophora a free-swimming con- tractile unbranched stem (hydrosoma),. which is rarely provided with simple lateral branches. The upper end of the hydro- soma is frequently dilated to the form of a flask (pneumatophore), and contains an air chamber [pneumatocyst] (fig. 185). In every case there is a central space in the axis of the stem in which the nutritive fluids are kept in constant motion by the contractility of the walls and by the move- ments of the cilia. The air sac or pneu- matocyst at the apex of the hydrosoma is connected to the chamber which contains it by radial septa, and in many cases attains a considerable size (Physalicb). It func- tions as a hydrostatic apparatus, and in those forms, which have a long spiral hydrosoma (Physopkoridce}, serves to keep the body in an upright position. In some cases the gaseous contents can escape freely by one or more openings. The appendages which are attached to the spirally twisted bilaterally symmetrical stem and whose cavities communicate with that of the stem are of at least two kinds —(1) The polypoid nutritive animals with their tentacles ; (2) the medusoid sexual buds. The nutritive Polyps (hydranths) are simple tubes provided with a mouth, and never. FIG. 186.—A portion of the stem and appendages of Halhtemma tergestlnum. St, Stem ; D, hy- drophyllmm; T, dactylozooid; Sf, tentacle of the latter ; Wg, female, Mg, male, gonophores. Gegenbanr. -'None Beitriige zur Kenntniss tier ; Nova Tom. XXVII., 1859. R. Leuckart, •• Zoologische Untersuchungen," Giessen. 185S. R. Leuckart. •
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
