1889.] NATURAL SCIENCES OF PHILADELPHIA. 119 



Upon the outside of the column is a thin cuticle (PL VII, fig. 6, cu.) 

 similar to what occurs in Z. sociulus and Z. flos-marinvs. Andres 1 

 considers this to be merely a differentiation or hardening 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 

 that tissue. Below this cuticle comes a layer of mesogloea for which 

 Andres' term subcuticula may be employed. The distinction between 

 the cuticle and this layer has been overlooked by most authors. It 

 was recognized by Kolliker, 2 however, who believed it to be a por- 

 tion of the cuticle. Andres recognized its true nature considering 

 it simply a continuation of the mesoglcea. 



Below the subcuticula is the ectoderm (PL VII, fig. 6, ec.) which 

 forms a layer 0"08 mm. in thickness. It is not continuous, 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 ectoderm of this species is the presence in 

 it of zooxanthella?. In adult actinians these structures are usually 

 confined to the endoderm, but I have observed them in the ectoderm 

 in free-swimming larva?, in which layer they also occur according to 

 H. V. Wilson 3 in the embryos of the coral Manicina. It is possible 

 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 conditions for the 

 growth of the parasitic alga?. The ectoderm thus buried in the 

 mesogloea evidently corresponds with what Kolliker, in the admira- 

 ble account he has given of the zoanthan mesogloea, 4 terms " eine 

 zusammenhangende Schicht driisenartiger Korper" and which he 

 believed to correspond to the ectoderm. 



The endoderm consists of low cells containing numerous zoo- 

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

 margin to the upper row of tubercles, is a single strong sphincter 



1 A. Andres. On a new genus and species of Zoanthina malacodermata ( Pan- 

 ceria spongiusa, sp. n.) — Quart. Journ. Micros. Sci. N. S. Vol. xvii. 1887. 



2 Kolliker. Icones Histologics. Leipzig. I860. 



:i //. V. Wilson. On the development of Manicina areolata. Journal of Mor- 

 phology. Vol. II. 1888. 



*A Kolliker, loc. cit. 



