. The Cambridge natural history. Zoology. CALYPTOBLASTEA LEPTOMEDUSAE 275 of them have been shown to produce numerous free-swimming Medusae by gemmation; but at present there is no reason to suppose that in these forms there is any extensive modification of the aboral extremity of the zooid to form such a liighly specialised organ as the float of Pelagoliydra. The affinities of Pelagohydra are not clear, as our knowledge of the characters of the Medusa is imperfect; but according to Dendy it is most closely, related to the Corymorphidae. Mar- gelofsis belongs to the Bougainvilliidae. Order IV.
. The Cambridge natural history. Zoology. CALYPTOBLASTEA LEPTOMEDUSAE 275 of them have been shown to produce numerous free-swimming Medusae by gemmation; but at present there is no reason to suppose that in these forms there is any extensive modification of the aboral extremity of the zooid to form such a liighly specialised organ as the float of Pelagoliydra. The affinities of Pelagohydra are not clear, as our knowledge of the characters of the Medusa is imperfect; but according to Dendy it is most closely, related to the Corymorphidae. Mar- gelofsis belongs to the Bougainvilliidae. Order IV. Calyptoblastea—Leptomedusae. The hydrosome stage is characterised by the perisarc, which not only envelops the stem and branches, as in many of the G-ymnoblastea, but is continued into a trumpet-shaped or tubular cup or collar called the " hydrotheca," that usually affords an efficient pro- tection for the zooids when retracted. No solitary Calyptoblastea have been discovered. In the simpler forms the colony consists of a creeping hydro- rliiza, from which the zooids arise singly (Clytia Johnstoni), but these zooids may give rise to a lateral bud which grows longer than the parent zooid. The larger colonies are usually formed by alternate right and left budding i'rom the last-formed zooid, so that in contrast to the Gymnoblast colony the apical zooid of the stem is the youneest, and not the oldest, zooid pm. ahydrociadmm J ^ ' ^ , , , . of a dried specimen of Plumu- of the colony. In the branchmg colonies the axis is frequently com- posed of a single tube of perisarc, which may be lined internally by the ectoderm and endoderm tissues formed by the succession of zooids to the branches by gemmation. Such specimen ( laria profunda. Gt, Gono- theca ; He, the stem of the hydrocladium with joints {]); lit, a single hydrotheca; N, nematophores. Greatly en- larged. (After Nutting.) that have a stem is given rise said to be. Please note that these
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
