INTRODUCTION. 5 



removed for a short time into the full rays of the tropical sun, or kept for 

 hours in diffuse light, when all the polyps expanded to their fullest degree, 

 and seemed tingling with vitality ; but in general, like most other corals 

 and actinians, the polyps seem most vigorous in the shade and at night time. 

 Detailed experiments on the phototropism of coral polyps are much needed. 



When the colonies were exposed to strong sunlight for a time, in a limited 

 quantity of water, bubbles of oxygen gas were seen to form within the cavity 

 of the polyps and then to escape through the mouth. The production of 

 oxygen is without doubt dependent upon the metabolic activity of the uni- 

 cellular algae or Zooxanthellse present in great numbers throughout the 

 endodermal tissues (p. 22). The filamentous green algae found perforating 

 the skeleton (p. 38) would doubtless have a similar activity, but the intensity 

 of the light reaching their chloroplasts must be much less than that falling 

 upon the more superficial Zooxanthellae. 



The oxygen produced from the combination of the physiological activities 

 of the plant and animal was ample to keep a limited quantity of water in a 

 proper state of aeration ; and once the colonies were established the water in 

 which they were living remained perfectly clear and sparkling without the 

 addition of other plant life. Generally it was unnecessary to renew the sea- 

 water, but from time to time fresh water was added to replace that lost by 

 evaporation. To make certain that the water contained sufficient calcareous 

 salts in solution to enable the young polyps to build up their skeleton it 

 was partly changed every few days. Like the adult polyps, the developing 

 individuals contained many Zooxanthellae within the endoderm. 



For food the polyps were given small living worms, pieces of molluscs, 

 crabs, or fish. These they took readily by first extending their tentacles 

 and later their mouths over them. Sometimes a hapless polychaete would 

 attempt to creep over a colony, when it was quickly killed and ingested ; other 

 small worms, wriggling with great vigor, were unable to free themselves when 

 dropped upon a colony. The tentacles of the polyps fastened upon the 

 annelid at one or more places, and its movements soon began to weaken, 

 as if the creature were paralyzed. Bach polyp immediately around the 

 worm opened its mouth widely, while those more distant extended their per- 

 istomes towards it, as if all were moved by a common impulse. As many as 

 half a dozen polyps would thus seize hold of a small worm at one time. 

 Naturally a difficulty arose when two polyps commenced to engulf the same 

 worm from opposite ends. The unfortunate annelid soon reached a state 

 of tension between the polyps, and the peristome of each of the latter was 

 drawn outwards to an unusual degree. For over an hour a worm was 



