560 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



peripheral part is usualh' more vertical, and bear.s the tentacles at its rim. During full expansion 

 the disk may be raised centrally to the extent of 4 mm. beyond the tentacular zone, and in this 

 condition it is conical in shape, and swollen a little at the stomodival region; the walls are very 

 thin and transparent, and permit of the arrangement of the mesenteries being followed. 



The stomodieum often protrudes a little, in which condition vertical ridges and furrows can 

 be clearly seen, six on each side. In pol3-ps partially exposed at the surface of the water, the 

 mouth becomes enlarged and circular in outline, to such an extent that the interior of the polvp 

 below the short stomodanim is visible. 



The color of the polyps throughout is a lighter or darker shade of brown, due to the color of 

 the chromoplastids in the endodermal zooxanthelhe. The ectoderm itself is colorless and trans- 

 parent, as can easily be seen when living tentacles are examined under the microscope; hence 

 the whiteness of the tip and tubercles on the tentacles, and of the lining of the stomoda?al wall. 

 When the tentacles are contracted they become verj^ dark brown, except at the tips, the densitj^ 

 in coloration resulting from the more closely aggregated unicellular alga>. The margin of the 

 peristome is at times a bright iridescent green. Where colonies occur on the underside of 

 some rock, or in such a position that little light reaches them, they may be colorless throughout, 

 or show all gradations from the normal intensity toward a bleached condition. 



The species is very favorable for the study of lateral columnar gemmation, and different 

 stages have been already described (p. 501). A single polyp, or maybe two, arises from the 

 upper portion of the coluuin wall of another polyp, and in turn gives origin to other buds. The 

 perithecal portions of the mesenteries of the parent seem altogether independent of the mesen- 

 teries of the bud, but four or five cost* of the parent corallite are continuous with the same 

 number of the bud. Very rarely a bud may arise on the disk of a large polyp in such a way that 

 the two are surrounded liy a conmion tentacular sj'stem and a single column. 



The buds develop to a certain extent before any septal foi'uiation can be recognized; in one 

 case twelve mesenteries could be made out through the semitransparent tissues of the distended 

 bud, without any evidence of septa or even of tentacles. The lower side of the bud grows in 

 advance of the upper, hence the angle which the axis of the bud makes with that of the parent 

 polyp; the lower tentacles likewise arise in advance of the upper. The manner of growth of the 

 mesenteries has already been descrilied (p. 458). 



The colonies live well in aquaria, the numerous symbiotic algw in the endoderm serving to 

 keep the water aerated, without any artificial means or constant exchange. All the members of a 

 subcolony respond when one is irritated, but slightly later. On retraction, the disk is lowered, 

 and its peripheral portion and the tentacles are ranged along the inside wall of the calice, resting 

 on the septa (fig. -iS), so that the mouth and middle region of the peristome are alone visible; 

 the column wall is then drawn horizontally over the margin of the calice, the circular edge 

 nearly closing in the middle. 



The most usual living condition appears to l)e one of moderate expansion, with the 

 tentacles erect or overhanging. Mesenterial filaments may be extruded, though very rarely, 

 through the mouth or other part of the disk, and have been observed displ'aced to such a 

 degree as to enter the tentacular cavities. They can be again indrawn. On narcotization with 

 menthol the tissues shrink somewhat, and the mouth becomes widely open and circular. 



Anatomy and histologi/. — Throughout the whole of its length, extending from the outer row 

 of tentacles to its terminal proximal mcrgin, the column wall presents a uniform structure (figs. 

 52a, 69). The ectoderm is the best developed of the three layers, and comprises numerous 

 unicellular gland cells with the contents clear or feebly granular. They constitute almost the 

 entire layer, and are mostly- ovoid ia shape, and may extend the whole thickness of the laj'er. 

 The interstices between the cells are filled l)y ciliated supporting cells with rounded or oval 

 nuclei. Seen in surface view the ectoderm gives the ap])earance of a mosaic, the large 

 polygonal gland cells forming clear areas, limited by the supporting cells as a kind of matrix. A 

 few granular gland cells and small nematocysts are also present. 



At its proximal termination the ectoderm thickens somewhat, and histologically is verj' 

 sharplj' marked ort' fi-oni the skclctt)trophic ectoderm. The uiesogltfa is usually \ery narrow, but 

 becomes thicker in the region of the mesenteries. ai)i)earing triangular in transverse sections; 



