﻿96 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 47 



delicacy and transparency, well exemplified by the Astrangia dance 

 of the eastern coast of the United States. Where colonies of other 

 species are found growing in the absence of much light, the polyps 

 are colorless, and zooxanthellse are sparse or wanting. 



Other more brilliant colors of corals are produced in various 

 ways. Most are ectodermal, due either to fine, granular pigment 

 matter within the layer, or to some superficial deposit. Distinct pig- 

 ment cells may also occur, either in one or both layers of the body 

 wall ; at other times large pigment granules are more or less uni- 

 formly distributed, rendering the tissues dense and opaque. Experi- 

 ments are much needed to determine the part which the zooxanthellse 

 and other coloring agents play in the physiology of the polyps, and 

 also as to their adaptive significance. 



THE POLYP WALLS 



The external walls of coral polyps, represented by the column and 

 disk, are constituted of the usual three ccelenterate layers : an outer 

 ectoderm, an inner endoderm, and a middle mesoglcea. The polypal 

 walls are usually thin and delicate, when compared with the majority 

 of polyps of the closely allied actinians, the difference being depen- 

 dent on the degree of development of the middle layer or mesoglcea. 

 The ectodermal and endodermal epithelia are found to vary com- 

 paratively little in height throughout the Anthozoa, so that any 

 diversity in thickness of the polypal wall as a whole is determined by 

 that of the mesoglcea. Support for the polyp is afforded by the 

 skeleton in corals, hence there is no necessity for any special develop- 

 ment of the middle layer, such as gives a certain degree of firmness 

 to many actinians. In general the mesoglcea of coral polyps is a mere 

 separating lamella ; where, as in the mesenteries, it may attain con- 

 siderable thickness, it is usually a clear homogeneous substance, 

 though in some species connective tissue cells occur sparsely. 



Both the ectoderm and endoderm are richly provided with unicellu- 

 lar gland cells, which give out copious supplies of mucus when the 

 polyps are irritated. An ectodermal muscle layer, the fibers either 

 longitudinally or radially arranged, has been found only on the 

 tentacles and disk, while the endodermal circular musculature of the 

 column is usually only feebly developed. In some of the larger 

 species the latter constitutes a diffuse endodermal sphincter, but never 

 becomes so concentrated as to form a circumscribed or mesoglceal 

 sphincter such as occurs in many actinians (bunodactids, sagartids). 



On complete retraction of the polyp the upper part of the column 



