xiv ZOANTHARIA EDWARDSIIDEA 375 



tentacles of a living Siderastraea a genus with Zooxanthellae, 

 the tentacles at once close upon them and prevent their escape. 

 The general conclusion seems to be, therefore, that the Madre- 

 porarian Corals feed upon small animals in much the same way 

 as the Sea-anemones, whether they have Zooxanthellae or not, 

 but that in general they feed only at night. 



Age. It is known that Sea-anemones kept in an aquarium 

 and regularly fed will live for a considerable number of years with- 

 out showing signs of weakness or failing health. Daly ell kept 

 in an aquarium a specimen of Actinia mesembryanthemum, which 

 lived for sixty-six years and then died a natural death ; and 

 specimens of Sagartia, still living, are known to be about fifty 

 years old. 1 The unnatural conditions of life in an aquarium may 

 have favoured the longevity of these specimens, and it would not 

 be reasonable to conclude from these records that the average life 

 of a full-grown Anemone on the rocks is more than thirty or 

 thirty-five years, and perhaps it is a good deal less. 



As regards the Madreporarian Corals, we know but little con- 

 cerning their duration of life. An examination of any living 

 coral reef is sufficient to convince an observer that the power of 

 asexual reproduction of the colonial forms is not unlimited ; that 

 colonies, like individuals, have a definite span of life, and that 

 they grow old, senile, and then die a natural death if spared in 

 their youth from accident and disease. Mr. Gardiner has 

 calculated that the duration of life in solitary Corals like Flabellum 

 is about twenty-four years, in colonial forms such as Goniastraea, 

 Prionastraea, Orbicella, and Pocillopora, from twenty -two to 

 twenty-eight years. 



Order I. Edwardsiidea. 



This order contains only a few genera and species of small 

 size living in shallow water in various parts of the world. In 

 external features they closely resemble several genera of the 

 Actiniaria, particularly those belonging to the family Hal- 

 campidae. The distinguishing character of the order is to be 

 found in the system of mesenteries. In all the species only 

 eight mesenteries are complete, namely, the first two pairs of 

 protocnemes, and the two pairs of directives (Fig. 163, 2), 



1 Ashworth and Annandale, Proc. Boy. Soc. Edinb. xxv. 1904, p. 11. 



