290 



COELENTERATA HY~ROZOA 



considerable variation, but are rseful characters in distinguish- 

 ing the Trachomedusae from the Leptomedusae. -The tentacles 

 are situated on the margin of the umbrella, and are four or eight 

 in number or, in some cases, more numerous. The gonads are 

 situated as in Leptomedusae on the sub-umbrella aspect of the 

 radial canals. 



In Gonionema murbachii the fertilised eggs give rise to a free- 

 swimming ciliated larva of an oval shape with one pole longer 



and narrower than the other. The 

 mouth appears subsequently at the 

 narrower pole. The larva settles 

 down upon the broader pole, the 

 mouth appears at the free extremity, 

 and in a few days two, and later 

 two more, tentacles are formed (Fig. 

 138). 



At this stage the larva may be 

 said to be Hydra-like in character, 

 and as shown in Fig. 138 it feeds 

 and lives an independent existence. 



Fig. 138. Hydra-like stage in the . \ . 



development of Gonionema mur- From its body-Wall buds arise which 



bachii. One of the tentacles is separa te from the parent and give 



carrying a worm (IF) to the , c . r m # 



mouth. The tentacles are shown rise to similar Hydra -like indi- 



very much contracted but they viduals> An asexua l generation thus 



are capable of extending to a > 



length of 2 mm. Height of zooid gives rise to new individuals by 



about 1 mm. (After Perkins.) ^_ , ,1 -i j n 



K ' gemmation as 111 the hydrosome of 



the Calyptoblastea. The origin of the Medusae from this Hydra- 

 like stage has not been satisfactorily determined, but it seems 

 probable that by a process of metamorphosis the hydriform 

 persons are directly changed into the Medusae. 1 



In the development of Liriope the free- swimming larva 

 develops into a hydriform person with four tentacles and an 

 enormously elongated hypostome or manubrium ; and, according 

 to Brooks, it undergoes a metamorphosis which directly converts 

 it into a Medusa. 



There can be very little doubt that in a large number of 

 Trachomedusae the development is direct, the fertilised ovum 

 giving rise to a medusome without the intervention of a hydro- 

 some stage. In some cases, however (Geryonia, etc.), the tentacles 



1 H. F. Perkins, Proc. Acad. Nat. Sci. Phil. Nov. 1902, p. 773. 



