CCELENTERATA 469 



firmer than the other layers. This is accordingly described as the 

 supporting layer (mesoderm). If we imagine a longitudinal axis drawn 

 through the centre of the body cavity, all the body parts will be arranged 

 symmetrically around this axis. The polyp, therefore, with all other 

 Ccelenterata, displays a radiate or radially symmetrical type of structure, 

 'like the echinoderms (see p. 459). 



E. Reproduction. 



(a) If the polyps are supplied with an abundance of food, it will be seen 

 after a few days that a small excrescence is developed on the body of 

 one or other of the animals, and into this the body cavity is continued. 

 It grows rapidly in length ; a mouth and tentacles are developed at its 

 free end ; finally, it separates from the body of the old polyp, and thus 

 gives rise to a young animal. This process reminds one of the budding 

 of plants, and is therefore described as budding or gemmation, the 

 excrescence itself being termed a bud. 



(b) If, on the other hand, the polyps are allowed to go without food 

 for a considerable time, or if the water in which they live dries up, or 

 'the weather gets too cold (in spring, autumn, or winter), small excres- 

 cences are again formed on the body, each of which this time contains 

 an egg. When ripe this delicate egg drops off, but, beiflg surrounded by 

 a strong chitinous envelope (see p. 308), it can be attacked neither by 

 frost nor drought. This is of the highest importance to an animal which 

 would be killed by the cold of winter or by the drying up of the water 

 in which it lives. At the same time, the production of eggs of such 

 minute size, which can be easily wafted away by the wind, is an excel- 

 lent means for disseminating the species. We can now also understand 

 why these eggs are produced exactly at the time when food is scarce, the 

 weather cold, and waters liable to dry up. 



CLASS I.: JELLY FISHES AND HYDROIDS (HYDROZOA). 



THE polyps in this division have no oesophageal tubes, and the body 

 cavity is not subdivided by mesenteries. (See Anthozoa, p. 473.) 



ORDER I. : LOBED MEDUSAE (ACALEPH^E). 



The Common Jelly-Fish (Aurelia aurita). 

 (Diameter of disc up to 6 inches.) 



A. Occurrence. 



ANYONE who has seen the sea must have noticed the peculiar jelly-fish 

 among the varied inhabitants of its teeming waters. Among these 



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