MEMOIRS OF THE NATIONAL ACADEMY OE SCIENCES. 581 



green, arranged in parallel or radiating rows, while the verruca' along the septal ridges are gray. 

 The green may predominate to such an extent as to give a decided tinge to the colony as a whole. 



White mesenterial filaments can be extruded through any part of the superficial tissues, and 

 quantities of mucus are given out on rough handling or preservation. 



Anatomy and histology. — The column wall is delicate and deeply folded in preserved material. 

 The ectoderm is a broad layer in which large, clear gland cells are by far the main constituents; 

 in many places extruded mucus can be seen adhering. Supporting cells appear to merely serve 

 as lines of separation between the closely arranged gland cells. In the lower region of the 

 ectoderm the cells contain finely granular matter, very irregularly distributed. In some spots 

 the granular matter may be wholly absent, and the clear contents extend the whole thickness of 

 the layer; in others it is deposited but a short distance from the mesoglceal boundary, and gives 

 the appearance of interstitial cells, each with its own rounded nucleus; again, in restricted 

 areas, the cells may be wholly granular as far as the outer surface of the ectoderm. Long, narrow 

 nematocysts occur here and there. In tangential sections toward the periphery of the ectoderm 

 a very regular polygonal arrangement is presented by the gland cells, the supporting cells occu 

 pying the interstices. The outer filiation is obvious in most preparations. 



The mesogloea of the column wall is thin and nearly homogeneous; included cells occur but 

 rarely. The endoderm is, like the ectoderm, highly glandular, and in addition contains many 

 zooxanthellse. A delicate circular musculature is developed, but presents no indication of 

 forming a special sphincter in the terminal region. 



The tentacles exhibit a broad ectoderm, and at intervals along- the sides and at the apex 

 are peripheral zones of nematocysts, all of the long, narrow 7 variety, with a close spiral thread. 

 A distinct zone of nerve fibrils is present toward the discal termination, and the ectodermal and 

 endodermal musculatures are both well represented. 



Histologically the disk differs in no important respect from the column wall; the endodermal 

 musculature is of the weakest character, while the presence of an ectodermal muscle layer is 

 not determinable with certainty. The stomodseum is oval, and of considerable vertical length; 

 in retracted polyps it is deeply folded both longitudinally and transversely, so much so that in 

 some transverse sections it appears for half a dozen times. The very pronounced vertical 

 ridges noted amongst the external characters are not so marked a feature in transverse sections, 

 especially when the stomodseum is open. The ectoderm exhibits the same histological structure 

 all the way round; the mesogbea is usually thicker at the positions corresponding with the 

 insertion of the mesenteries, and in some places regular ridges are formed. Very few nema- 

 tocysts occur in the stomodseal ectoderm; the cells are nearly all ciliated supporting cells, while 

 granular gland cells are distributed at intervals. 



The mesenteries are only divisible into complete and incomplete pairs, there being no regular 

 cyclical arrangement; further, the alternation of complete and incomplete pairs is by no means 

 uniform: sometimes two or three successive alternations may occur, and at other times all the 

 pairs will be complete for some distance. There is little doubt that the incomplete pairs are but 

 new pairs in process of growth which will ultimately become complete, and can not, therefore, 

 be regarded as representing a distinct order. No directives occurred among a large number of 

 mesenteries examined. 



The mesoglceal lamina of the mesenteries is very variable in character, being in some places 

 broad and in others narrow. The foldings for the longitudinal musculature, as a rule, are only 

 feebly developed, but on one member of a pair they may be very pronounced, while scarcely 

 distinguishable on the other. The oblique musculature is weak above, but stronger below. The 

 mesenterial epithelium is broad, and comprises mainly clear gland cells; very often the clear 

 secretion is preserved in the act of extrusion. 



The mesenterial filaments are of the usual type, but certain of the mesenteries become greatly 

 convoluted below, the filament following the convolutions all the way; in such cases the 

 convolutions in section constitute a very close, irregular mass (fig. 148). 



The septal invaginations are in nearly all cases entoccelic, but occasionally a short invagination 

 may occur without any associated mesenteries being discoverable, so that probably the septa may 

 appear somewhat in advance of their corresponding mesenteries and thus be exocoelic. 



