. A textbook of invertebrate morphology [microform]. Invertebrates; Morphology (Animals); Invertébrés; Morphologie (Animaux). 94 INVERTEBRATE MORPHOLOGY. In some forms, such as Hydractinia, the free swimming embryo when it settles down becomes converted into a Hut plate-liko expansion without mouth or tentacles, from which, as a bud, the first hydranth arises. If, however, the ovum develops directly into a medusa, as in the Trachymedusse and NarcomedusjB, the breaking through of the mouth and the formation of tentacles takes place while the embryo is still free-swimming, and the stage so produ
. A textbook of invertebrate morphology [microform]. Invertebrates; Morphology (Animals); Invertébrés; Morphologie (Animaux). 94 INVERTEBRATE MORPHOLOGY. In some forms, such as Hydractinia, the free swimming embryo when it settles down becomes converted into a Hut plate-liko expansion without mouth or tentacles, from which, as a bud, the first hydranth arises. If, however, the ovum develops directly into a medusa, as in the Trachymedusse and NarcomedusjB, the breaking through of the mouth and the formation of tentacles takes place while the embryo is still free-swimming, and the stage so produced may resemble closely a free-swimming hydranth, as in Canoctantha, or may, by the great development of mesogkjea at the extremity opposite the mouth, assume a rather globular form, as in Liriope. As the tentacles develop and the bell becomes differentiated by the extension laterally, as it were, of the embryo, the velum arises at the margin of the bell. At this time the coelenteron is a flattened cavity extending to the margins of the bell, but later it becomes obliterated along four lines, and the obliteration of the cavity extending, the radiating and circular canals and the gastric cavity alone persist, a layer of endoderm-cells sometimes joining them and representing the obliterated portion of the coelenterou, though often this also disappears. In the Anthomedusffi and Leptomedusse, in which the medu- ste arise by budding from the polyps, the buds are at first tubular outgrowths of the body-wall (Fig. 48, A). The ecto- derm at the tip of the bud thickens, depressing the central portioii of the endoderm (Fig. 48, B), and on the appearance of a cavity in the thickened ectoderm, the subumbrellar cavity, the central endoderm pushes out into the cavity, carry- ing with it the ectoderm covering it and forming the manu- brium (Fig. 48, C). In this stage the bud, though still lacking mouth and tentacles, is comparable to the polyp stage of the medusa of direct development at l
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