CCELENTEBATES. 29 



uality. Amongst these Coelenterates we find two chief types, 

 the so-called Medusa or Jellyfish and the Polyp. These two 

 totally different animals are one and the same, the medusa being 

 a sexual form of the asexual polyp. There is thus produced a 

 most remarkable phenomenon, known as the alternation of 

 generations — that is, the alternation of a sexual and an asexual 

 form of the same creature. 



The class Coelenterata contains the Corals {Actinozoa), Dead- 

 men's Fingers (Octactinia), Sea-anemones (Hexactinia), the 

 polypoid and medusoid Hydrozoa, and the Ctenophora. A 

 polyp is a simple tubular body fixed at the posterior end and 

 pierced by an oral opening at the free end, the mouth being 

 surrounded by a circle or several circles of tentacles. Polyps 

 may reproduce by male and female cells — spermatozoa and 

 ova — or by budding. All colonial forms are produced by the 

 latter process. A medusa or jellyfish is free-swimming, and 

 consists of a flattened or arched gelatinous disc, that we so 

 often see 'floating on the top of the sea. From underneath 

 this disc there hangs down a stalk, the manubrium, at the 

 free end of which opens the mouth. Tentacles may be de- 

 veloped around the mouth and edge of the disc. Here in the 

 medusa we find that the distinctly defined mouth leads into a 

 canal that runs up the stalk and enters a cavity in the disc, the 

 stomach, from which canals run out to the edge of the disc, 

 where they form a circular canal surrounding it. 



A medusa may be compared to a flattened polyp. In the 

 hydroid polyp stock reproduction takes place by budding, so 

 that the individual colony increases ; but every now and then a 

 modified bud forms — a medusoid bud — in the place of a polyp. 

 This bud breaks off and floats away as a medusa, which becomes 

 sexually mature, producing ova : these ova hatch into free-swim- 

 ming larvae that settle upon some rock or stone, when each larva 

 turns to a polyp which creates a colony by repeated gemmation. 

 Thus we get an alternation of a fixed asexual and a free sexual 

 generation. 



