XIV ZOANTHARIA LARVAE— FOOD 373 



summer months, and remain small and relatively inconspicuous 

 in the colder weather ; but British Sea-anemones, when kept in 

 an aquarium and regularly fed, will breed nearly all the year 

 round. The corals of the tropics living in warmer water of a 

 more regular temperature show considerable variety in their 

 breeding habits. Thus Duerden found that colonies of Favia, 

 Manicina, Siderastraea and Porites are fertile at nearly all times, 

 whereas colonies of Madrepora, Oriicella and Cladocora were 

 rarely so. In nearly all cases the fertilisation is effected, and 

 segmentation of the ovum occurs within the body of the parent, 

 the young Zoantharian beginning its independent life as an 

 oval or pear-shaped ciliated larva. 



There are a great many cases among the Actiniaria in which 

 the embryos are retained within the coelenteron, or in special 

 brood pouches of the parent (p. 379), until a stage is reached 

 with twelve or more tentacles. 



The oval or pear-shaped larva swims about for a few days or 

 hours, and then settles down on its aboral end. In swimming, 

 the aboral end is always turned forwards. In the larva of 

 Lebrunia coralligens and Rhodactis sancti -thomae, a distinct 

 sense organ has been observed upon the aboral extremity, and 

 a similar but less distinct organ on the larva of Actinia equina. 

 These organs are of considerable interest, as they are probably 

 the only specialised sense organs known to occur in the 

 Zoantharia. 



The larvae of Zoantharia present, as a rule, very little 

 variation from the type described, and live but a short time if 

 they fail to find a suitable place for fixation. The colour is 

 usually white and opaque, but in some species the endoderm may 

 be coloured yellow by Zooxanthellae (cf pp. 86, 125). 



The larvae of the Cerianthidea, however, are remarkable 

 and exceptional. After the larva of these animals has passed 

 through the gastrula stage, a certain number of mesenteries and 

 tentacles are formed, and it rises in the water to live a pelagic 

 life of some duration. This larva is known as Arachnactis, and 

 is not unfrequently found in the plankton. 



The character of the food of the Zoantharia varies with the 

 size of the zooids, the occurrence of Zooxanthellae in the 

 endoderm, and local circumstances ; but in general it may be 

 said to consist mainly of small living animals. 



