80 



INVEItTEBRATE ANIMALS. 



they do), tliey 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 zodids of the Lucernarida or hidden-eyed Meduscp, on the other 

 hand, the swimmingdiso or umbrella is destitute of any marginal shelf or veil ; 

 the radiating gastro-vascular canals are never less than eight in nuinber, and 

 they split up into numerous branches, which unite to form an intricate net- 

 work ; lastly, tlie marginal l^odies are concealed I'rom view by a kind of hood. 



There still remains anotlier family of the Lucernarida (viz., Rhizo- 

 stomidie) 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 zouids is somewhat different. In these, as in KJdzo- 

 stiiiiKi (hj 44) the !,euei itive zooid is much Hke 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- 

 1 irella itself does not exhibit any difference 

 IS compared with those already described, 

 but the ova are produced in a genital 

 ( tvity which is placed on the under sur- 

 f ice 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 

 hnd their nearest allies in the Sertulariaiis. 

 The (jraptolitidce are without a single liv- 

 ing representative, and their antiquity is, 

 indeed, very high, since it is doubtful if 

 they ever jiass above the gToiip of rocks 

 known to geologists as the Silurian formation. The most typical 

 forms of the groui) (fig. 45) agree with the living Sertularians in 

 having a hoi-ny jjolyjiary, and in h.aviiig the polypites protected by 

 little horny cups or hydrotheca?, all springing from a common stem 

 or coenosarc. The typical Graptolites, however, differ from all 

 known Sertularians in the fact that the hydrosoma was not iixeil 

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

 exliiliit -.1 vc)y anomaliuis and remarkable structure, termed the 

 "solid ;ixis" (tig. 45, B). This is a peculiar, fibrous, apparently 



I n 1 ti\ z il rf 

 ; / I 1 u I 

 (Aft. 1 G ssf ) 



