CCELENTER ATA S YDROZO A. 13? 



attached to the disc, four canals proceed to the margin 

 of the bell. All these open into a circular canal which 

 surrounds the margin. The ova and sperm cells are pro- 

 duced either in these canals or in the manubrium. 



199. The third kind are similar to the preceding, but 

 become detached and lead an independent existence, 

 swimming about by the alternate contraction and ex- 

 pansion of their discs. A mouth is developed at the 

 outer extremity of the manubrium, and the polypite be- 

 comes self-feeding. The mouth of the bell is closed, with 

 the exception of a central aperture, by a delicate mem- 

 brane called the veil. A number of tentacles are 

 produced round the margin. As soon as fertile ova are 

 developed, the creature dies. 



These bell-shaped gonophores are commonly called sea- 

 jellies or sea-medusa, and were formerly believed to be 

 distinct animals. The margin of the bell in some groups 

 is adorned with a number of bright spots, which are 

 supposed to be either organs of sight or hearing. These 

 are called the naked-eyed medusae. They vary much 

 in size, some of the smaller kinds being not larger than 

 the head of a pin. Many of them have brilliant colours, 

 and are very beautiful. They are described by the late 

 Professor Edward Forbes as " gorgeous enough to be the 

 diadem of the smallest of the sea-fairies, and sufficiently 

 graceful to be the night-cap of the tiniest and prettiest of 

 mermaids." They are generally phosphorescent, and con- 

 tribute largely to the luminosity of the sea. The ova 

 developed by the medusa?, instead of reproducing animals 

 of the same kind, grow into fixed plant-like organisms, 

 similar to those from which the gonophores were de- 

 tached, thus furnishing an example of what has been 

 called, "alternation of generations;" that is, "an in- 

 dividual is not at all like its mother, but exactly re- 

 sembles its grandmother." 



200. The Sertularidae (Lat. sertum, a wreath), or 

 sea-firs, are more generally known than any of the com- 

 pound Hydrozoa. The polypary is much branched, often 



