80 



IXVEETEBRATE ANIMALS. 



they do), they do not form an intricate network ; lastly, the marginal bodies 

 are simply placed in an uncovered situation on the margin of the disc. In the 

 reproductive zooids of the Lucernarida or hidden-eyed Medusae, 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 net- 

 work ; lastly, the marginal bodies are concealed from view by a kind of hood. 



There still remains another family of the Lucernarida (viz., Rki:o- 

 stomidce) 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 zooids is somewhat diiferent. In these, as in Rhizo- 

 sioma (fig. 44), the generative zooid is much like those just men- 

 tioned ; but the umbrella is destitute of 

 marginal tentacles ; and in place of a 

 single central polypite, there hangs from 

 the under surface of the umbrella a com- 

 plex tree-like mass, the branches of which 

 end in, and are covered by, small poly- 

 pites and club-shaped tentacles. The um- 

 brella 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 Graptolitid,^. 



We may here notice very briefly a group 

 of extinct organisms which almost certain- 

 ly belong to this class, and which probably 

 find their nearest allies in the Sertidariaiis. 

 The Graptolitidce are without a single liv- 

 ing representative, and their antiquity is, 

 indeed, very high, since it is doubtful if 

 they ever pass above the group of rocks 

 known to geologists as the Silurian formation. The most typical 

 forms of the group (fig. 4.5) agree with the living Sertularians in 

 having a horny polypary, and in having the polypites protected by 

 little horny cups or hydrothecse, all springing from a common stem 

 or ca'.nosarc. The typical Graptolites, however, differ from all 

 known Sertularians in the fact that the hydrosoraa was not fixed 

 to any solid object, but was permanently free. Most of them, also, 

 exhibit a very anomalous and remarkable structure, termed the 

 " solid axis " (fig. 45, B). This is a peculiar, fibrous, apparently 



Fig. 44. — Generative zooid of 

 Rhizostonm p^Umo, reduced 

 in size. (After Gosse.) 



