. Text-book of zoology for junior students. Zoology. 68 IXVEKTEBRATE AXIMALS. than the ordinary Lydrotliecj?. As a general rule, each capsule in the Sertidarida contains a number of the reproductive liuds, -which are all borne upon a central axis which luns \ip the middle of the capsule, anil is derived from the cnenosarc. In one di\isi(jn of this group—often described as a separate order, under the name of Vaiiijianuhirida—some points of difference are observable. In the typical Sertularians the little cups or hydrothecse for the polypites are placed on the sides of the liranches, and they ar
. Text-book of zoology for junior students. Zoology. 68 IXVEKTEBRATE AXIMALS. than the ordinary Lydrotliecj?. As a general rule, each capsule in the Sertidarida contains a number of the reproductive liuds, -which are all borne upon a central axis which luns \ip the middle of the capsule, anil is derived from the cnenosarc. In one di\isi(jn of this group—often described as a separate order, under the name of Vaiiijianuhirida—some points of difference are observable. In the typical Sertularians the little cups or hydrothecse for the polypites are placed on the sides of the liranches, and they are not stalked (fig. 34, a'), whilst the rejjroductive elements are pioduced in fixed buds. In the Camprimdarida, on the other hand (fig. 34, ?)), the hydrotheca^ are supported upon stalks, and are placed at the ends of the branches, whilst the generative buds are usually detached to lead an independent existence. In these forms the reproductive zooids or gonophores start as simple buds; but they become gradually developed into free-swimming meilusoids, such as have been 1 lefore alluded to. Each medusoid consists of a little transparent glassy bell, from the under surfjice of which there is suspended a modified polj'pite, in the form of a mamdjrium (fig. 36, a). The whole or- ganism swims gaily through the water, priijjelled by the contractions of the bell or disc ; and no one would suspect now that it was in any ^^'ay related to the fixed jilant-Iike zoophyte from which it was originally builded off. The cen- tra,l polypite is furnished with a mouth at its distal end, and the mouth opens into a digestive sac. From the proximal end of this stomach proceed four radiat- ing canals which extend to the margins of the bell, where they all open into a circular vessel which runs round the mouth of the tiell. Fi'om the circumfer- ence of the liell hang also a number of delicate extensile filaments or tentacles; and the margin is further adorned with a series of briglitly coloured s
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1885
