MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 575) 



moved about 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 discal fission, pairs of small oral apertures are 

 frequently observed very close to one another, each exhibiting l>ut a few stomodseal 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 through the mouth upon slight disturbance, and can be again withdrawn.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 arc 

 ordinarily distinguishable. Large quantities of mucus were emitted on preservation of the 

 colonies. The action of the superficial ciliation can be readily observed by 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. 



Anatomy and histology. — The column wall is a thin layer throughout. In the ectoderm 

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

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

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

 mesenteries, and the end.od.erm is much narrower than the ectoderm, and its cells contain 

 zooxanthelhe. 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, 

 entocoelic and exocoelic chambers. 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 deeply-staining 

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

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

 thin layer without zooxanthellffl. 



The disk differs 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 stomodival ectoderm is thrown into very deep vertical folds, each ridge corresponding 

 with the point of attachment of a mesentery, and supported by a long, narrow, mesoglcea! axis. 

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

 surface; very 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, 131). 



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 appearand 1 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, <f seq.). 



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

 figs. 129 and 130, no regularity is maintained, and the opposite face may also be deeply sinuous. 

 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 exoscpta ever really occur. 



As shown in tie'. 129, the skeletotrophic endoderm varies greatly in character in the different 

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

 over the septal invaginations. 



