. Un texte-livre d'invertébrés à la morphologie. Les invertébrés. CCELENTERA TYPE. 77 iBg à son extrémité l'ouverture à la bouche. Ceci mène à une cavité centrale, l'coelenteron, couché dans le fond de la cloche, et à partir de cette amende ou canaux qui se propagent vers le bord, où, dans certains cas, ils sont unis par un canal circulaire qui va tout autour de la cloche à cette région. À la marge de la cloche tentacules sont habituellement fixés, et organes des sens, présentant souvent envisager- en mesure de la complexité de la structure, se trouvent dans les intervalles entre les tentacules ou à leurs bases. Ces formes

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. A text-book of invertebrate morphology. Invertebrates. TYPE CCELENTERA. 77 iBg at its extremity the mouth-opening. This leads into a central cavity, the coelenteron, lying in the substance of the bell, and from this pouches or fine canals radiate out towards the rim, where, in some cases, they are united by a circular canal which runs completely round the bell at this region. To the margin of the bell tentacles are usually attached, and sense-organs, presenting frequently consider- able complexity of structure, are found in the intervals be- tween the tentacles or at their bases. These forms, known as medusce, are, as a rule, free-swimming, propelling them- selves through the water by vigorous contractions of the bell. All Cnidaria, whether of the polyp or medusa form, pos- sess in their tissues peculiar elements altogether unrepre- sented in the Sponges. These are the nematocysts or so-called thread-cells. Each consists of an oval or spherical cyst (Fig. 38, c) with a membranous wall and fluid contents, the wall being pro- * "^ longed at one end into a long, exceed- ingly delicate, hollow, thread-like fila- ment sometimes furnished with spines at its base, and, in. an undisturbed cyst, is invaginated, and coiled up in the interior (Fig. 38, t). These nematocysts are pro- duced by and enclosed within special Fig. 38. — Nematoctst cells known as cnidoblasts (en) lying Cell op Physalia. principally in the ectoderm, and in their ^^ ^ °^™* "j'^^'' most highly developed form differen- ^n = cnidoblast. tiating below into a supporting stalk (s) n = nerve prolongation. which rests upon the outer surface of « = supporting process, the mesogloea. From the outer extremi- ~ "^^ ty there projects beyond the surface of the ectoderm a short hair-like process, the cnidocil (cT), and, in addition, an exceeding- ly fine process of some length (n) is given off at the junction of the stalk with the cyst-containing portion of the cell. These two processes are supp

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• bookcentury1800
• bookdecade1890
• bookpublishernewyo
• bookyear1894