290 COELENTERATA — HYDROZOA chap. 



considerable variation, but are useful 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. 

 1.38). 



At this stage the larva may be 

 said to be Hyd7'a-\T^e in character, 

 and as shown in Fig. 138 it feeds 



FIG. 138. ^^^I^e stage in the '^^^^ ^^^"^^ ""^^ independent existence, 

 development of Oonionema mur- From its body-wall buds arise which 



'^X^'Tl^'i^^'Z ^^P^^-t^ f^^"^ the P'^re^t and give 

 mouth. The tentacles are shown rise to similar Hyclra-\i\<ie indi- 



very much contracted, hut they „-a 1„ a i j.' ii 



are capable of extending to a ^iduals. An asexual generation, thus 

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



about 1 mm. (After Perkins.) .. ■ ji i , \. 



gemmation as m 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.^ 



In the development of Liriope the free -swimming larva 

 develops into a hydriform person with four tentacles and an 

 enormously elongated hypostome or manulDrium ; 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 (Gcryonia, etc.), the tentacles 



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



