CORALS AND CORAL REEFS VAUGHAN. 



207 



expand as in the colony represented by plate 17. This expansion was 



instigated by placing the colony in a vessel in a shady (not really 



dark) place, where it would not be shaken, and then feeding it with 



a little beef juice through a pipette, or by giving it a small bit of 



meat, usually crab flesh or fish. The tentacles at the end of the 



colony to which the food was offered would begin to appear, and the 



stimulus was transmitted 



to other members of the 



colony, until after a short 



time the surface of the 



specimen would remind 



one of a beautiful open 



flower. This condition of 



a coral colony seems to 



signify that it is hungry 



and is ready to capture 



food. 



Special mechanisms of 

 corals for catching food 

 are greatly developed. 

 They comprise, as fol- 

 lows: (1) The nemato- 

 cj'^sts, the stinging cells 

 and their coiled threads, 

 which occur in the ecto- 

 derm, the outer layer of 

 the soft tissue and its 

 modifications, on the ten- 

 tacles, the oral disk (be- 

 tween the tentacles and 

 the mouth), the sides of 

 the polyps, and also on 

 the mesenterial filaments. 

 (2) The entire ectodermal 

 surface is ciliate, the cilia 

 in response to certain 

 stimuli beating toward 

 the oral apertures; in re- 

 sponse to others, beating toward the periphery. (3) The outer sur- 

 face secretes mucus in which particles may be embedded, the mucus 

 moving under the influence of the beat of the cilia toward the oral 

 apertures or toward the periphery, according to the nature of the re- 

 sponse to the stimulation. (4) The tentacles are active and effective 

 in capturing food. (5) The mesenterial filaments, which in many 

 species of corals can be extruded through the column Avails, in some 

 instances capture food. 



Fig. 4. — Nematocysts of Maeandra areolata 

 (Linnaeus), vert highly magnified. After L. 

 Agassiz from drawings by Burkhardt and D. 



SONHEL. 



