ANEMONES AND CORALS 51 



fully expanded Anemone, whose numerous tentacles, plentifully 

 supplied with thread - cells, also probably serve to keep many 

 unwelcome visitors away, and also may at times help in the 

 killing or paralysing of prey. 



An even more intimate association is exhibited in some species 

 which invariably contain numerous " yellow cells," which have 

 been shown to be really minute one-celled plants thriving in 

 mutual partnership or symbiosis with the animal cells of the 

 Anemone. While the carbonic acid gas given off by the animal 

 cells is useful to, and absorbed by, the minute plants, the liberated 

 oxygen given off and the starch formed by these vegetable cells 

 are of great value to the Anemone. In some of the Anemones 

 both ova and sperms are developed within the same animal, 

 while in other genera the sexes are distinct. The ova undergo 

 their early stages within the parent, and a ciliated planula is set 

 free. 



The White Stony Corals, or Madreporaria, belong, like the 

 Anemones, to the order Zoantharia; and, as may be seen from 

 the photograph of an expanded specimen of the little Devon- 

 shire Coral (Caryophyllia Smithii) facing p. 53, a living simple, 

 solitary Coral somewhat resembles in outward appearance an 

 Anemone. On top of the body there are a series of tentacles, 

 and a central disk within^ne circle of tentacles, in the midst of 

 which is a small mouth. A coloured tissue, like that of the out- 

 side of the tentacles and disk, covers the outside of the body, 

 and the disk is marked with lines which appear to radiate from 

 the mouth, and if touched will contract slightly so that a hard 

 structure is felt beneath it, made up of a number of thin, ver- 

 tically placed plates, with their edges upwards. Between these 

 plates, or septa, as they are called, spaces exist (the interseptal 

 spaces) in which there is a process of the under part of the disk, 

 the mesenteric folds. If the disk be removed, the tops of the 

 numerous septa will be seen covered with a filmy structure, and 

 between each pair of plates a soft mesenteric fold. Just under 

 the opening of the mouth, and in the middle of the top of the 

 Coral, there may be a hard projection called the columella ; or, 

 should this not exist, the stomach cavity occupies its place. Where 

 a columella is present the stomach is immediately above it, but 

 in either case the stomach has the mesenteries radiating on all 



