82 INVERTEBRATE ANIMALS. 



From the above description it will be evident that there is 

 considerable resemblance between the so-called " hidden-eyed" 

 3fech6sce, or the reproductive zoOids oi many oi the Ijueernarida, 

 and the medusiform gonophores of so many of the Sydrozoa, 

 as well as the true Discophora or naked-eyed Medusae. The 

 differences, however, between them are these : The swimming- 

 disk of the naked-eyed MeduscB and of any medusiform gono- 

 phore is furnished at its mouth with an internal shelf or veil ; 

 the radiating gastro-vascular canals are very rarely more 

 than four in number, and, should they subdivide (as in rare 

 cases they do), they do not form an intricate net-work; lastly, 

 the marginal bodies are simply placed in an uncovered 

 situation on the margin of the disk. In the reproductive 

 zooids of the Lucernarida or hidden-eyed Medusm, on the 

 other hand, the swimming-disk or umbrella is destitute of 

 any marginal shelf or veil ; the radiating gastro-vascular 

 canals are never less than eight in number, and they spHt 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 Xiucernarida (viz., 

 Hhizostomidce) 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 zoBids is somewhat different. 

 In these, as in JRhizostoma, the generative zoSid is much 

 like those just mentioned ; 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 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 surface of the 

 umbrella. 



Sub-class Graptolitid^. — Before leaving the Hydrozoa, it will be as Tvell 

 to notice very briefly a group of extinct organisms which certainly belong to 

 this class, and which probably find their nearest allies in the Serttdariam. 

 The Graptolitidce are without a single living representative, and their anti- 

 quity 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 agree with the living Sertularians in having a 

 horny polypary, and in having the polypites protected by little horny cups 

 or hydrothecie, all springing from a common flesh or ecenosaro. The typical 

 Graptolitea, however, differ from all known Sertularians in the fact that the 

 hydrosoma was not fixed to any solid object, but was permanently free. 



