126 THE BERMUDA ISLANDS. 



ening of the external layers of the mesogloea, but I cannot 

 agree with this view. It is a clearly defined layer external to 

 the mesogloea, and appears quite different in composition and 

 behavior to staining fluids from tliat tissue. Below this cuticle 

 comes a layer of mesogloea for which Andres's term subcuticula 

 may be employed. The distinction between the cuticle and 

 this layer has been overlooked by most authors. It was rec- 

 ognized by Kolliker', however, who believed it to be a portion 

 of the cuticle. Andres recognized its true nature, considering 

 it simply a continuation of the mesogloea. 



Below the subcuticula is the ectoderm (PI. 11, fig. 6, ec), 

 which forms a layer 0'08 mm. in thickness. It is not contin- 

 uous, however, but is divided into more or less cubical masses 

 by columns of mesogloea extending from the general mass of 

 that tissue to the subcuticula. A peculiar feature of the ecto- 

 derm of this species is the presence in it of zooxanthellse. In 

 adult actinians these structures are usually confined to the en- 

 doderm,but I have observed them in the ectoderm in free- 

 swimming larvae, in which layer they also occur according to 

 H. V. Wilson' in the embryos of the coral Manidna. It is pos- 

 sible that their presence in the ectoderm of M tuberculata is due 

 to the thick cuticle and subcuticula preventing a rapid aeration 

 of the ectoderm cells, and so, by favoring the accumulation to 

 a certain extent of carbon dioxide, producing favorable condi- 

 tions for the growth of the parasitic algse. The ectoderm tlius 

 buried in the mesogloea evidently corresponds with wliat 

 Kolliker, in the admirable account he has given of the zoan- 

 than mesogloea,' terms "eine zusammenhangende Scliiciit 

 drusenartiger Korper" and which he believed to correspond to 

 the ectoderm. 



The endoderm consists of low cells containing nunierous zoo- 

 xanthellse. In the upper part of the column, extending from 



' KolliKer. /cones Hislologicie. Leipzig. 186-5. 



2 H, V. Wilson. On the Development of Manicina areolala. Journal of Mor- 

 phology. Vol. II. 1888. 



3 A. Kolliker, loc. cit. 



