MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 079 



moved al)out very rapidly within the interior of the parent, and numbers would be shot out 

 together on irritation. 



In the increase in size of the colony by partial di.scal fission, pairs of small oral apertures are 

 freijuently observed very close to one another, each exhibiting- but a few stomoda-al ridges 

 (eight to ten), in such a way as to leave little doubt that the two have resulted from the 

 division of a single large aperture. 



The polyps do not readily respond to irritation, but retract and expand slowly. White 

 mesenterial filaments, with parts of the mesenteries to which they are attached, are extruded 

 through the mouth upon slight disturbance, and can be again withdrawn. On irritation the 

 filaments may protrude in great profusion through any part of the disk, but no apertures are 

 ordinarily distinguishable. Large ([uantities of nmcus were emitted on preservation of the 

 colonies. The action of the superficial ciliation can be readily observed liy placing some light 

 particles on the middle of the disk; the particles are carried slowly outward, and for some dis- 

 tance down the edge-zone. 



An((f(iiiii/ (111(1 hhtohigy. — The column wall is a thin layer throughout. In the ectoderm 

 unicellular mucous glands, with clear contents, arc very abundant, and less so small narrow 

 nematocysts. No pigmented granular cells are seen, the tissues, as noted amongst the external 

 characteristics, being nearly transparent. The mesogloea is thin, except where united with the 

 mesenteries, and the endoderm is much narrower than the ectoderm, and its cells contain 

 zooxanthelhv. Very delicate endodermal muscle fibers can also be detected, but in the upper 

 region no concentration of the fibrils occurs in any way suggestive of a sphincter muscle. 



In sections of retracted polyps the tentacles are frequently found introverted within the 

 entocffilic and exocoelic chambei's. The ectoderm is a much thickened layer, crowded with long 

 narrow nematocysts, with a very distinct spiral thread. They are more numerous apically, and 

 at places corresponding with the lateral thickenings; a few gland cells with deei)ly-staining 

 contents occur, and also developing nematocysts in the deeper situations. An extremelv weak 

 ectodermal and endodermal musculature can be distinguished, and the endoderm is a comparatively 

 thin layer without zooxanthelLt. 



The disk difl'ers but little from the column wall, except that all the layers are somewhat 

 thicker, and both the ectodermal and endodermal musculatures are better developed; small 

 nematocysts also occur peripherally. 



The stomoda^al ectoderm is thrown into very deep vertical folds, each ridge cori'esponding 

 with the point of attachment of a mesentery, and supported by ii long, narrow, mesoglceal axis. 

 The layer is very broad, and the well-defined nuclear zone is situated a little below the free 

 surface; A-erj' large nematocysts, showing the internal thread, occur here and there, and in the 

 deeper regions others can be traced in various stages of development (figs. 129, ]31). 



Two principal orders of mesenteries are present — complete and incomplete, each pair 

 embracing, as it were, a septum. At irregular intervals are pairs of much shorter mesenteries, 

 which appear to be in process of development, but not representing a distinct order or cycle. 

 The order of appearance of the mesenteries for this species has been already described; larval 

 polyps are regularly hexameral (fig. 132), but this is lost after fission is instituted (p. 502, et .scfj.). 



The mesoghea on the face bearing the retractor muscle is usually folded, but, as shown by 

 figs. 120 and 130, no regularity is maintained, and the opposite face may also be deeply simious. 

 The mesenterial filaments as a rule are present on all the mesenteries, but on the youngest 

 members they may be only incipient. 



Septal invaginations are found within all the entocoelic chambers, and occasionally one is seen 

 within an exocoelic chamber; but in these latter instances higher sections usually reveal a pair of 

 small mesenteries, so that it may be doubted whether exosepta ever really occur. 



As shown in fig. 129, the skcletotrophic endoderm varies greatly in character in the difi'erent 

 regions, being greatly thickened and vacuolated in the lower and peripheral areas, and narrow 

 over the septal invaginations. 



