. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 656 Comparative Animal Physiology like process by eversion (Fig. 248). There is usually a cap or operculum covering that region of the undischarged nematocyst through which the thread is ejected. The nematocysts are produced within interstitial cells either at their final site or at some distance from this site, to which they move by ameboid activity or passive transportation. A cell containing a nematocyst is known as a cnidoblast or nematocyte. The portion of the cnidoblast con- taining the nematocyst eventual
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 656 Comparative Animal Physiology like process by eversion (Fig. 248). There is usually a cap or operculum covering that region of the undischarged nematocyst through which the thread is ejected. The nematocysts are produced within interstitial cells either at their final site or at some distance from this site, to which they move by ameboid activity or passive transportation. A cell containing a nematocyst is known as a cnidoblast or nematocyte. The portion of the cnidoblast con- taining the nematocyst eventually comes to occupy a superficial position and usually develops at its outer end a bristle-like projection, the cnidocil. The latter is imbedded in a small crater-like elevation on the cnidoblast also often difi^erentiates trichite-like supporting rods in its peripheral regions and often, too, a fibrillar network associated with the capsule of the nemato- cvst and extending proximally from it. The nematocvsts of coelenterates are divisible into two major groups, the spirocysts of the Zoantharia, which are acid-staining bodies possessing peculiar adhesive threads, and the basic-staining nematocysts proper. The latter are widely distributed through the whole phylum. They are of many types in the characters of their discharged threads. In some, , the volvents, the thread is closed at its tip and forms a tightly coiled filament on discharge, wrapping. Fig. 248. An undischarged (a) and a discharged (b) nematocyst located in its cnidoblast. The spine-like projection of the cnidoblast is the cnidocil. Redrawn from Sedgwick. itself about bristles or fibers of organisms in whose presence they are dis- charged. Most other types have open tips and are believed to penetrate prey, injecting into them a toxic substance. The threads of these latter .types com- monly possess an armature in the form of three spiralling rows of spines which serve effectively to anchor the thread into
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