CCELENTERATA. l8l 



The mesenteries bear -masses of nettling cells and the reproductive 

 bodies. The eggs or young finally escape through the mouth to the 

 outside world. 



Sea anemones may also bud, as in Hydra, or divide longitudinally 

 into daughters of equal size. 



Anemones are well supplied with muscular fibres, both longitudinal 

 and circular, atlhough they have no power of locomotion. When 

 low tide leaves them bare or when disturbed, they have the power 

 of contracting their muscles in such a way as to withdraw all the 

 tentacles and to close up the whole free oral surface. In such a 

 contracted position they have nothing of the beauty which they 

 show when extended. 



197. The Corals and Sea-fans. Closely akin to the anemones, 

 both in form and in internal structure, are the corals. They differ 

 in the fact that the corals develop a supporting skeleton of limy 

 (corals proper) or horny (sea-fans) substance, and in the fact that 

 they are usually colonial while the anemones are solitary. The 

 colonies are formed by budding, as in the Hydrozoa, but in the 

 corals much more of the hard material is deposited, and great masses 

 of carbonate of lime (limestone) are formed. The coral reefs and 

 coral islands found in the tropical seas are the products of these 

 small polyps. 



Coral masses differ very much in form. Some are slender and 

 branching, as in the sea-fans and tree corals; others are spherical 

 and massive, as the brain corals; still others are flat. If budding 

 and secretion take place in all directions, and the different polyps 

 grow at much the same rate, a form like the brain coral will result. 

 What conditions of budding and growth would produce forms 

 like the branching or tree corals? See figures in various text-books. 



198. Ctenophora or "Comb-jelly": a Library Exercise. Refer to 



the figures in three or four advanced, well-illustrated texts. What 

 do you see about these foms that would seem to ally them with the 

 other coelenterates ? In what respects are they similar to the jelly- 

 fish? In what respects dissimilar? What gives the name Cteno- 

 phora (comb-bearer) to the group? How does the gastro- vascular 

 cavity compare with that of the other coelenterates ? 



199. The Body Organs and their Functions. Consisting 

 as these animals do of a cylindrical body with a single 

 central cavity, it is easy to see that the organization is 

 still simple. All the organs are simple modifications of 



