374 COELENTERATA ANTHOZOA chap. 



Sea-anemones kept in an aquarium will readily seize and 

 devour pieces of raw beef or fragments of mussel that are offered 

 to them ; but they may also be observed to kill and swallow the 

 small Crustacea that occur in the water. "When a living animal 

 of a relatively small size comes within range of the tentacles, 

 it appears to be suddenly paralysed by the action of the nemato- 

 cysts and held fast. The tentacles in contact with it, and 

 otliers in the neighbourhood but to a lesser extent, then bend 

 inwards, carrying the prey to the moutli. The passage of the 

 food through the stomodaeum is effected partly by ciliary, and 

 partly by muscular action, and the food is then brought to the 

 region of the mesenteric filaments where it is rapidly disinte- 

 grated by the digestive fluids they secrete. Any unsavoury or 

 undigested portions of the food are ejected by the mouth. 



Very little is known concerning the food of the Madreporariau 

 Corals. Many investigators have noticed that the zooids of 

 preserved specimens very rarely contain any fragments of animal 

 or plant bodies that could possibly be regarded as evidence of 

 food. It is possible that many Corals derive a part, perhaps in 

 some cases a considerable part, of their nourishment from the 

 symbiotic Zooxanthellae (pp. 86, 125) which flourish in the 

 endoderm ; but it is improbable that in any case this forms the 

 only source of food supply. The absence of food material in 

 the cavities of the zooids may perhaps be accounted for by the 

 fact that nearly all the Corals are fully expanded, and therefore 

 capable of catching their food only at night. Corals are usually 

 collected during the daytime, and therefore during the period 

 of rest of the digestive organs. 



It is true that nearly all Corals do exhibit Zooxanthellae in 

 their endoderm, but there are some species from whicli they 

 are nearly or wholly absent, such as Astrangia solitaria and 

 Phyllangia americana on the West Indian reefs,^ and the 

 Pocilloporidae. The absence of any signs of degeneration in 

 the tentacles or digestive organs of those corals with Zooxanthellae 

 as compared with those without them suggests, at any rate, that 

 the Zooxanthellae do not supply such a large proportion of the 

 food necessary for the support of the colonies as to warrant any 

 relaxation of the efforts to obtain food by other means. Mr. 

 Duerden found that when living Annelids are placed upon the 

 ^ Duerden, Mem. Acad. Washington, viii. 1902, p. 437. 



