144 ZOOLOGY 



SECT. 



ends, into the jelly of the ex-umbrella in the form of "tentacle- 

 roots " (t.r). 



As to the position of the reproductive organs, there is the 

 same difference between the two sub-orders of Trachylinse as 

 between the two sub-orders of Leptolinse. In the Trachymedusse 

 the gonads (Fig. 99, gori) are developed in the course of the radial 

 canals : in the NarcomedusaB (Fig. 100) they lie in the manubrium, 

 sometimes extending into pouch-like offshoots of its cavity. 



There is always a well-developed velum, which, as in Fig. 100, 1. 

 may hang down vertically instead of taking the usual horizontal 

 position. In the Narcomedusa3 the manubrium is short ; in the 

 Trachymedusse it is always well developed, and is sometimes (Fig. 

 99, B) prolonged into a long, highly contractile peduncle, having 

 its inner surface produced into a tongue-like process (tg) which 

 protrudes through the mouth. 



The simplest case of the development of Trachylinaa is seen in 

 sEginopsis, one of the Narcomedusae. The oosperm gives rise to 



FIG. 102. Larva of JEginopsis. m. mouth; t. tentacle. (From Balfour, after Metschnikoff.) 



a ciliated planula, which forms first two (Fig. 102), then four 

 tentacles, and a mouth, hypostome, and stomach. The larva of 

 ^Eginopsis is thus a liydrula, closely resembling the corresponding 

 stage of Tubularia. After a time the tentacular region grows out, 

 carrying the tentacles with it, and becomes the umbrella of the 

 medusa. Thus the actual formation of the medusa from the 

 hydrula of ^Eginopsis corresponds precisely with the theoretical 

 derivation given above (p. 127). It will be seen that in the present 

 case there is no metagenesis or alternation of generations, but that 

 development is accompanied by a metamorphosis that is, the egg 

 gives rise to a larval form differing in a striking manner from the 

 adult, into which it becomes converted by a gradual series of 

 changes. 



Metagenesis is, however, not quite unknown among the Trachy- 

 linse. In a parasitic Narcomedusa (Cunina parasitica) the planula 



