88 INVERTEBRATE ANIMALS. 
situation on the margin of the disc. In the reproductive zodids 
of the Lucernarida or hidden-eyed Meduse, on the other hand, 
the swimming-disc or umbrella is destitute of any marginal 
shelf or veil; the radiating gastro-vascular canals are never 
less than eight in number, and they split up into numerous 
branches, which unite to form an intricate network; lastly, the 
marginal bodies are concealed from view by a kind of hood. 
There still remains another family of the Lucernarida (viz., 
Rhizostomide) in which the reproductive process is carried on 
in the same way as in the forms we have just described, but the 
structure of the reproductive zodids is somewhat different. In 
these, as in AAzzostoma (fig. 30), the generative zodid is much 
like those just mentioned; but the umbrella is destitute of mar- 
ginal tentacles ; and in place of a single central polypite, there 
hangs from the under surface of the umbrella a complex tree- 
like mass, the branches of which 
end in, and are covered by, small 
polypites and club-shaped tentacles. 
The umbrella itself does not exhibit 
any difference as compared with 
those already described, but the ova 
are produced in a genital cavity 
which is placed on the under sur- 
face of the umbrella. 
SUB-CLASS GRAPTOLITIDA,—Before 
leaving the Hydrozoa, it will be as well ° 
to notice very briefly a group of extinct 
organisms which certainly belong to this 
class, and which probably find their 
nearest allies in the Sertularians. The 
Graplolitide are without a single living’ 
representative, and their antiquity is, 
indeed, very high, since it is doubtful if 
they ever pass above the group of rocks 
Fig. 31.—Morphology of Grapto- known to geologists as the Silurian for- 
Pe, Net e ieee 7 ai mation. ‘The most typical forms of the 
sagtarin, enlreed’ 2 C°2%- group (see fig. 31, 2 and 3) agree with 
drothecee on one side only; 3, the living Sertularians in haying a horny 
Diplograpsus pristis, showing polypary, and in having the polypites 
hydrothece: on both sides. protected by little horny cups or hydro- 
thecze, all springing from a common 
flesh or comnosare. The typical Graptolites, however, differ from all 
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