Outlines of zoology . Fig. 71—Bougainvillea.—After Allman. A. A small piece of a hydroid colony. p,^ Peri-sarc ; »/,, medusoid bud; h,, hydranth or polyp head. B. A medusoid ; vta.^ manubrium; , radial canal; j., sense- organ. the bell also bears tentacles, usually hollow, and abundantlyfurnished with stinging cells (Fig. 65, 3). On the convex surface of the bell the ectoderm formssimply an epithelial layer; on the concave surface it isdifferentiated into muscle cells on the velum, the manu-brium, and the tentacles, nerve cells at the base of thevelum, and stinging cells on the tentacles. T
Outlines of zoology . Fig. 71—Bougainvillea.—After Allman. A. A small piece of a hydroid colony. p,^ Peri-sarc ; »/,, medusoid bud; h,, hydranth or polyp head. B. A medusoid ; vta.^ manubrium; , radial canal; j., sense- organ. the bell also bears tentacles, usually hollow, and abundantlyfurnished with stinging cells (Fig. 65, 3). On the convex surface of the bell the ectoderm formssimply an epithelial layer; on the concave surface it isdifferentiated into muscle cells on the velum, the manu-brium, and the tentacles, nerve cells at the base of thevelum, and stinging cells on the tentacles. The endodermis ciliated; it lines the food canal, and extends also into the TYPES OF CCELENTERA A MEDUSOID. 151. tentacles. The mesogloea forms a thickened jelly, presentmore especially on the convex (ex-umbrellar) surface. The mouth opens into the canal of the manubrium, whichleads to the central cavity of the dome. With this a varyingnumber of unbranched radial canals communicate; theseopen into a marginal circular vessel, which communicateswith the cavities of the tentacles. A plate of endoderm liesin the mesogloea between the radial canals. Digestion isintracellular, and probably goeson throughout the whole of this gastro-vascular system. The movements of the bellare caused by the contractionsof the ectodermic muscle cells. The nervous system consistsof a double ring of nerve fibresaround the margin of the these are associated gang-lionic cells, which apparentlycontrol the muscular contrac-tions. Sense organs may be present,in the form of eyes, at thebase of the tentacles (Ocellatse),or in the form of auditoryvesicles developed as pits in thevelum (Vesiculatae). The reproductive or
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
