FEEDING REACTIONS IN CORAL POLYPS. 429 



amount of foreign material was to be seen, which could easily be 

 extracted with a small forceps, without any damage to the polyps. 

 The contents of the gastric cavity of 20 polyps consisted besides 

 of undeterminable matter (detritus) of the following foreign 

 organisms or parts of these: living diatoms (found in 12 polyps), 

 diatom scales (found in 9 polyps), parts of higher algae, usually in 

 a partially decayed state (found in 4 polyps), foraminifera (found 

 in I polyp), spicules of sponges (found in 5 polyps), parts of the 

 stalks of hydroids (found in 2 polyps), a living nematode (found 

 in I polyp), a dead larva of a polychaet worm (found in I polyp), 

 parts of appendages or segments of the body of different smaller 

 crustaceans (found in 10 polyps), shells of small bivalve mol- 

 lusks (found in 3 polyps). Often also nematocysts or parts of 

 these occur among the food-remnants and in the polyps which live 

 in symbiosis with zooxanthellae invariably also these yellow algae 

 are to be found in the gastric cavity. Only very few polyps, 

 when examined immediately after being collected, do not contain 

 anything in their gastric cavity. 



It is an interesting fact that in those polyps of Astrangia in 

 which zooxanthellae occur in the entoderm cells, these algae are 

 always found in the remains of the food in the gastric cavity. 

 These algae are here in different stages of decomposition, owing 

 to their being digested by the polyps. In this respect the polyps 

 of Astrangia possessing zooxanthellae agree closely with reef- 

 corals, in which the symbiotic algae are also found in a partially 

 digested state in the mesenterial filaments. 5 



There are, in general, two opinions concerning the food of reef- 

 corals. One of these opinions was first put forward in a number 

 of publications by Gardiner (1899, 1902, 1902-03, 1904^). This 

 author found that the zooxanthellae form a large proportion of the 

 food of all reef-corals, and maintains that many species of these 

 corals even feed entirely on their symbiotic algae. In a later 

 publication Gardiner (1912) states that zooxanthellae are largely 

 eaten by the coral-polyps when they require food, and further 

 mentions that it is supposed that they catch and digest the small 



6 After my studies in Woods Hole I made some observations on the feeding of a 

 few reef corals in the Bermuda Biological Station for Research. The polyps of 

 these corals (Isophyllia and Siderastrea) in the natural state invariably contain 

 partially decayed zooxanthella? in the digestive zone of the mesenterial filaments- 



