THE HYDROMEDUSAE 



be added either four (Sminthonema) or six (Geryones) interradial 

 tentacles. By further additions they may amount to more than a 

 hundred (Olindias) ; in Pectanthis they form sixteen bundles. 

 The tentacles are 

 often arranged in 

 two or more rows 

 in such a manner 

 that some take 

 origin, not from 

 the extreme mar- 

 gin of the bell, but 

 at a little distance 

 from it on the 

 exumbral surface ; 

 the endodermal 

 axis of the ten- 

 tacle still retains 

 connection with 

 the more central 

 endoderm, by 

 bending inwards 

 through the thick 

 exumbral meso- 

 gloea. This bent 

 axis, together with 

 bands of cnido- 

 blasts, which run 

 from the marginal 



FIG. 38. 



llread V men- 

 J 



of the disc centripetally in the outer surface of the jelly-like disc ; 

 six of these are perradial, six interradial, corresponding to the 

 twelve solid -larval tentacles, resembling those of Cunina ; fc, 

 dilatation (stomach) of the pseudo-manubrium ; I, jelly of the 

 disc ; p, pseudo-manubrium ; t, tentacle (hollow and tertiary, i.e. 

 preceded by six perradial and six interradial solid larval tentacles) ; 

 it, cartilaginous margin of the disc covered by thread cells; v, 

 velum. (From Lankester.) 



l'<i riiiuri n<i (Gcryonid) lumtuta (after Haeckel). o, nerve and 

 cnidoblast ring ; a', radial nerve and canal ; 6, tentaculocyst ; 

 c, circular canal ; e, blind centripetal canal ; g", ovary ; h, peronium 

 tioned to the point or cartilaginous process ascending from the cartilaginous margin 



of attachment of 

 the tentacle, give 

 rise to the char- 

 acteristic mantle 

 rivets or peronia. 



The musculature of the bell is of the usual type, except for the 

 great development of radial muscle bands along the course of the 

 radial canals through the subumbrella and pseudo-manubrium. 



In two out of the four families into which this group i& 

 divided (Petasidae, Trachynemidae) the general relations of parts 

 of the medusoid are of the type already familiar (Fig. 5) ; but 

 in the other two (Aglauridae, Geryonidae) the gastric cavity does 

 not lie in the subumbrella, but is situated at the distal end of the 

 apparent " manubrium " ; the latter is really a prolongation of the 

 subumbrella, solid except for the radial canals, and may be termed 

 a pseudo-manubrium. The mouth is generally surrounded by four 



