482 



CHARACTERS OF INVERTEBRATE ANIMALS 



Fig. 295. Development of Aurelia, enlarged 



a, The fixed stage (hydra-tuba), b and c, Transverse splitting of a to form 

 medusae, d and e, Young medusa, seen from the side and from below. 



of the year. The edge of the umbrella is lobed, the notches 

 between the lobes sheltering peculiar sense organs formed by the 

 modification of tentacles and covered by little lappets. Hence 



the term " covered- 

 eyed medusae " some- 

 times applied to these 

 forms, to distinguish 

 them from the' 'naked- 

 eyed medusae " of the 

 preceding class, in 

 which the marginal 

 sense organs are not 

 covered by such lap- 

 pets. These terms, 



hoWCVCr, WCrC not 

 U q nnil V rllOQf^n fnr if 



is only in certain cases 

 that the sense organs have to do with sight. A negative charac- 

 teristic of the jelly-fish included in this division is to be found in 

 the absence of the true velum (see p. 479). The most typical 



members of the group present two stages in 

 the life-history, as in Obelia (p. 480), i.e. (i) 

 a fixed hydroid stage, and (2) the free-swim- 

 ming medusa. The common British form 

 Aurelia may be taken as an example (fig. 

 295). Here the fixed stage is what is called 

 a Hydra-tuba, somewhat resembling a short, 

 broad Hydra in shape, though internally it 

 presents a difference in the presence of four 

 longitudinal folds which project into the diges- 

 tive cavity. Medusae are developed from it, 

 not by budding, but by a process of trans- 

 verse splitting, and when the process is far 

 advanced, the incipient medusae may be com- 

 pared to a pile of saucers. Sooner or later 

 these become detached and grow into the mature jelly-fish, by 

 which eggs are produced which complete the cycle of development 

 by becoming hydra-tubae. 



Some of the medusae belonging to the order have no fixed 

 stage, and in the case of certain other species (Haliclystus, 



Fig. 296. Lucernaria, enlarged 



