CORALS. 



115 



methods are asexual in their character, the last sexual. In reproduction by a fission 

 we find a simple voluntary self-division of a single individual into two or more second- 

 ary animals. In many reef-building corals this method of increase is most natural in 

 order to increase the size and style of growth of a colony of these animals. Among 

 the solitary forms like Metridium it is seldom found. A second mode of increase, 

 a reproduction by gemmation or budding, is much more common than that of fission 

 and is found in the solitary as well as the communal forms of Actinozoa. In the 

 formation of a bud we have a very simple method of reproduction. In such a case 

 there simply appears on one side of the base of the body, or, as in some genera, 

 on the disk surrounding the mouth, a small protuberance, which is a simple elevation 

 of the body walls. From this simple beginning of a bud we pass to a more developed 

 condition in which the protuberance has become a small coral animal attached to the 



Fig. 107, — Crambactis araiicat sea anemone. 



parent at one extremity which is its base, and with a free extremity furnished with a 

 mouth suiTOunded by a circle of tentacles in most respects identical with those of the 

 mother. The bud from the parent has every resemblance to the parent, and can live 

 independently although still attached, and drawing nourishment in part from her 

 through the base of attachment. 



All the Actinozoa reproduce by means of eggs. The ova pass through the condition 

 of a ciliated planula which is free-swimming and sometimes parasitic in its youth. 

 Phenomena similar to those of alternation of generations have been found in some 

 genera, but as a rule the development is direct from the egg to the adult. Special 

 features in the development of individual genera will be touched upon as we continue 

 our account of these animals. 



The Actinozoa are commonly divided into two great groups, easily distinguished 

 from each other, and known as, the Actinoida or Zoantharia, and Halcyonoida or Alcy- 

 onaria, which includes, roughly speaking, the reef-builders in the first instance and 



