. Outlines of zoology. Zoology. 144 CCELENTERA. exceptionally in Aurelia. Cyanea is often very large, "it may measure 7| ft. across the bell, with tentacles 120 ft. ; Chrysaora is her- maphrodite, and has diffuse sperm sacs even upon the arms. In the Rhizostomze, Cassiopeia and Pi/ema, the mouth is obliterated, and replaced by numerous small pores on the four double arms. Litcemaria and its allies are interesting sessile forms which have been compared to sexual Scyphistomas, that is, are re- garded as persistently larval forms. We may note here that Chun, while agreeing pro
. Outlines of zoology. Zoology. 144 CCELENTERA. exceptionally in Aurelia. Cyanea is often very large, "it may measure 7| ft. across the bell, with tentacles 120 ft. ; Chrysaora is her- maphrodite, and has diffuse sperm sacs even upon the arms. In the Rhizostomze, Cassiopeia and Pi/ema, the mouth is obliterated, and replaced by numerous small pores on the four double arms. Litcemaria and its allies are interesting sessile forms which have been compared to sexual Scyphistomas, that is, are re- garded as persistently larval forms. We may note here that Chun, while agreeing provisionally to the separation of the Acraspeda from the Hydrozoa, strongly opposes their association with the Anthozoa, basing his opposition especially on the existence of Scyphistomas of great simplicity { Spongicold).. Fig. 62 ——After Korotneff. Contrast between Hydrozoon Medusoids Scyphozoon MeduscB. Hydrozoon. (Craspedota.) The majority are small "swimming ; A flap or velum (craspedon) projects in- wards from the margin of the bell. No tainiolae, nor gastric filaments. A double nerve-ring around the margin. Naked sense organs either optic or audi- tory. Theyare usually derived from the skin, but the auditory sacs may be modified tentacles. Reproductiveorgans on the radial canals or by the side of the manubrium. The reproductive cells are usually derived from the ectoderm. With the exception of the Trachy- medusse, all arise as the liberated reproductive persons of Hydroid colonies. True Hydrozoa. SCVPHOZOON. (ACKASl'EDA.) Many are large "jelly-fish.'' No velum. (The velarium of Aurelia is a mere fringe, very inconspicuous in the adult, and not inturned.) In the Scyphistoma there are four tasniolse, from part of which the gastric filaments of the adult grow. Eigbt separate nervous centres be- side the sense organs, and a sub- umbrellar nervous plexus. Sense organs are modified tentacles, and probably have almost always a triple
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