. Biology . particularly abundant on thetentacles and on the hypostome, where they are arranged ingroups, usually one large one surrounded by a crown of smallerones. They are called nettle or stinging cells because of thepresence of a coiled thread which is thrown out when the cell isirritated. The tip of the thread contains a trace of poison, sothat minute animals struck by them are paralyzed and becomeaii easy prey for the tentacles and mouth. These cells, which 86 ORGANISMS OF TISSUES are sometimes called nematoblasts or cnidoblasts, thus performfunctions of offence and defence. Each stingi
. Biology . particularly abundant on thetentacles and on the hypostome, where they are arranged ingroups, usually one large one surrounded by a crown of smallerones. They are called nettle or stinging cells because of thepresence of a coiled thread which is thrown out when the cell isirritated. The tip of the thread contains a trace of poison, sothat minute animals struck by them are paralyzed and becomeaii easy prey for the tentacles and mouth. These cells, which 86 ORGANISMS OF TISSUES are sometimes called nematoblasts or cnidoblasts, thus performfunctions of offence and defence. Each stinging cell possesses a sensory hair or trigger,called a cnidocil, at the free end, and a thread-holding capsule,the nematocyst, within. The thread is formed from a cellgrowth which is spirally wound in the capsule, while capsule,and its contents are all formed by differentiation of a singlenucleated cell of the ectoderm. During growth of the capsuleand thread the young nettle cells first lie near the supporting. Fig. 36.—Diagrammatic figure of the cells from a small portion of the body wallof Hydra viridis; the ectodermal cells with nematocysts below (one with pro-truded thread), and the large vacuolated endoderm cells above. The symbioticalgae are grouped near the supporting lamella at the bases of the endoderm cells.(From Marshall and Hurst.) lamella in the region of the mouth, but they migrate duringthe period of their formation, and finally come to lie on the sur-face of the ectoderm around the mouth or on the tentacles andbody, the cnidocils ultimately projecting slightly beyond thesurface of the body in the surrounding medium (Fig. 36). 3. Nerve Cells.—The nerve cells of Hydra and the coelenter-ates represent the special differentiation of cells for the per- IT. /CJIZRTS COLLEGE UBRARYHISTOLOGY OF HYDRA 87 formance of the single function of irritability. All cells ofHydra, like all protoplasm, are irritable and respond to externalstimuli, but the nerve cells are especial
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Keywords: ., bookcentury1900, bookdecade1910, bookpublishe, booksubjectbiology
