Elementary text-book of zoology . elementarytextbo0101clau Year: 1884 ACTIXOZOA. 223 principally of the ectoderm, but also of the entoderm. Each Cnido- blast, from the contents of which a nematocyst is developed, possesses a fine superficial plasmatic process (Cnidocil), which is probably very sensitive to mechanical stimuli, and occasions the bursting1 of PIG. 168.—Group of nematocysts at the end of the tentacle of a Scyjihiftoma the capsule. Very frequently the Cnidoblasts are found thickly grouped to- gether at certain places, and form wart-like swellings or batteries (fig. 168). The di
Elementary text-book of zoology . elementarytextbo0101clau Year: 1884 ACTIXOZOA. 223 principally of the ectoderm, but also of the entoderm. Each Cnido- blast, from the contents of which a nematocyst is developed, possesses a fine superficial plasmatic process (Cnidocil), which is probably very sensitive to mechanical stimuli, and occasions the bursting1 of PIG. 168.—Group of nematocysts at the end of the tentacle of a Scyjihiftoma the capsule. Very frequently the Cnidoblasts are found thickly grouped to- gether at certain places, and form wart-like swellings or batteries (fig. 168). The differentiation of tissues and oi-gans also appears to have i cached a higher stage in the Cnidaria, in comparison with the Porifera, in which cnidoblasts have not hitherto been discovered. Sense cells, in particular, are found in the ectoderm, and these are not seldom grouped together as specific sense organs. Nerve cells and fibres are also present ; the latter often form a deeper layer of fibrous tracts beneath the superficial layer of the ec- toderm, With which they Stand FIG. 169.— Longitudinal section through the nerve ring of Charybdea. Sz> Sense cells in the ectoderm ; Gz, ganglion cells ; Nf, nerve . , . , , . -. , , fibres ; StJ, supporting lamella ; E, entoderm cells. pro- cesses of the sense cells. Amongst many Medusae (Craspedota and Charybdea) we find a single or double nerve ring near the edge of the disc, while in the Polyps (Actinia], the nerve fibres have a more irregular distribution (fig. 169).
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