1 6 SIDERASTREA RADIANS. 



Acropora (Madrepord], the hydrozoan Millepora, or the zoanthid Palythoa, 

 all owe their origin to the presence of unicellular algae with yellow chromo- 

 plasts within the endodermal cells.* 



Stanley Gardiner and Hickson have both drawn attention to the part 

 which commensal algae probably play in the nutrition of the polyp. The 

 organisms are actually inclosed within the endodermal cells, and there can 

 be scarcely any doubt that the polyp receives from them nutritive carbohy- 

 drates, produced in the course of the metabolic activity of the vegetable cells 

 under the action of sunlight ; further, the Zooxanthellae are constantly 

 increasing in number by fission into two and then four, and perhaps some 

 are utilized as food directly by the polyp. The liberation of free oxygen, 

 already shown to take place by the activity of the chlorophyll within the 

 algal cells, must also be of importance in the vital activities of the polyps. 



In many corals and actinians, however, Zooxanthellae are altogether 

 absent, hence their presence can not be considered as necessary to the life 

 of the polyp ; also, corals grown in the shade are colorless from absence of 

 the algae. Further, both actinians and corals will ingest almost any variety 

 of animal food that is offered them. 



REPRODUCTION. 



New polyps arise asexually as intercalary buds, usually at the point of 

 junction of three or more polyps, so that it is impossible to say whether one 

 polyp more than another is to be regarded as the parent, or as to how far 

 the structures in the bud arise in organic association with those of the older 

 polyps. Over the general surface of any colony bud polyps in different 

 stages of development are usually present, and for some time they remain 

 much lighter in color than the others, owing to a less growth of Zooxan- 

 thellae ; also, around the margin of colonies the addition of new polyps 

 is generally in rapid progress. These latter serve to enlarge the lateral 

 boundaries of the colonies, while the intercalary polyps occupy the spaces 

 produced as the coral grows in height, and, of course, in superficial area. 

 Instances of reproduction by the process known as fissiparous gemmationf 



*In his recent vice-presidential address before the Section of Zoology of the American Association 

 at St. Louis, Prof. C. W. Hargitt comes to the conclusion that the colors in coelenterates and many other 

 groups of invertebrates have probably no adaptive or protective significance, and are in main the result 

 of the metabolic activity of the animal. The rich profusion and beauty of color in coral polyps certainly 

 seems to have no protective or warning significance, but those due to the presence of commensal algie 

 have of course an altogether different physiological importance from the other colors. 



t The term is applied to a method of asexual reproduction occasionally found in gemmiferous colonies. 

 It appears as if an enlarged polyp simply divides into two. The method, however, is altogether different 

 from growth in fission colonies generally. ("The Morphology of the Madreporaria IV. Fissiparous 

 Gemmation." Ann. Mag. Nat. Hist., ser. 7, vol. xi, 1903.) 



