Elementary text-book of zoology, tr Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 ACTrNOZOA. 223 Fm' ie*-Gr°«P <* nomatocysta at th* end of the tentacle of a Scyphist ma principally of the ectoderm, but also of the entoderm. Each cnido- llast, 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 bursting of the capsule. Very frequently the cnidoblasts are found
Elementary text-book of zoology, tr Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 ACTrNOZOA. 223 Fm' ie*-Gr°«P <* nomatocysta at th* end of the tentacle of a Scyphist ma principally of the ectoderm, but also of the entoderm. Each cnido- llast, 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 bursting of 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 organs 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, a e 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 Ckaryldea. in connection ^z' Sense cells in the ectoderm; Gz, ganglion cells; Iff, nerve fibres ; Stl, supporting lamella; E, entoderm cells. through 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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