MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 559 



Each colon) is usually divisible into smaller subcolonies, in which a certain numDer of the 

 polyps are still unite. 1 one with another by the column wall. In any distinci subcolonj rarely 

 more than four or five polyps are united, branching at an angle varying from nearly a right angle 

 to about 15 . Sometimes a single polj p may be disconnected from all the others, or only two or 

 three may be in union. All stages in the formation of subcolonies, by the disappearance of the 

 intervening portion of the soft tissues, are presented. During the process the proximal part 

 of the column wall of two united polyps becomes constricted more and more, and finally the last 

 connecting strand bi'eaks down, and the polyps are completely isolated. The separation of 

 individual polyps is more frequent in some of the humbler, bushy colonies, while other colonies are 

 met with in which all the polyps are still united, no subcolonies being formed, or very sparsely. 



The part of the skeleton exposed by the withdrawal of the lower perithecal portion of 

 the column wall and skeletotrophic tissues is at first (dean and white, and covered with a very 

 thin epitheca; hut worm tub s, algse, etc., from the older, dead parts of the colony soon encroach 

 upon it. 



The column wall is cylindrical, and extends downward over the outside of the skeleton for a 

 varying distance in different polyps, hut rarely exceeding 5 mm. from the thecal edge. The surface 

 i- smooth and semitransparent, and is very distinctly ridged and grooved throughout its extent, 

 the ridges corresponding with the costse and the grooves with the internal attachment of the mesen- 

 terial continuations. The mesenteries seem to extend the full length of the column, hut sections 

 reveal that toward the lower extremity some become free from their inner attachment, or may dis- 

 appear altogether (tigs. 51, 54). When the polyps are expanded to their full extent, the column, 

 tentacles, and disk become removed some distance from the underlying skeleton, and the columnar 

 ridges and grooves are not very pronounced. The column may extend upward as much as 4 mm. 

 beyond the theca, and the disk, becoming conical, may add another 4 mm. On full retraction the 

 tissues adhere very closely to the corallum, following its elevations and depressions; above, the 

 column wall is overfolded, so as to cover the tentacles and greater part of the disk. On partial 

 retraction the column wall is sometimes constricted in a circular manner, just below the tentacular 

 zone. The, lower, terminal edge of the column of the lowest polyp in a subcolony is very sharply 

 defined, and the upper passes directly into the tentacles. 



The tentacles are in three or four cycles, hut beyond the first cycle of six the arrangement 

 is difficult to determine, as but small variations in size are exhibited, and the cycles are closely 

 arranged. The general appearance is that of two alternating cycles of nearly equal size. The 

 total number of tentacles varies from twenty-four to thirty-six; thirty and thirty-four are 

 most frequently counted. 



All the tentacles are rather broad at the base, and narrow towards the apex, which is slightly 

 swollen or knobbed. In the expanded condition the organs are usually overhanging, often one 

 cycle to a greater extent than the other. Sometimes the members of the outer cycle may be 

 reflected to such a degree as to be opposed to the column wall, while those of the inner cycle 

 remain erect. Occasionally the tentacles may be shorter and more swollen proximally, as in the 

 figure given by von Heider of C. cespitosa (1881)', but generally they are long and narrow. In 

 one or two instances a tentacle with a bifurcated apex has occurred, each half provided with a 

 swollen tip. 



The tentacular walls are very dedicate and transparent on full distention, bearing minute 

 white tubercles over the whole surface. Like the apical swelling, these are only thickenings 

 of the ectoderm, and on microscopic examination are seen to be batteries of stinging cells. The 

 tentacles correspond in position with the septa and costse, which are easily seen through the semi- 

 transparent tissues: the inner tentacles are placed over the larger septa, and the outer cycle over 

 the alternating smaller septa. The tentacular apex lias considerable adhesive power, holding 

 on to any body brought in contact with it; the distal part of the stem also tends to fold round 

 any object. The length of the innex-most tentacles during full extension may be as much as 6 mm. 



The disk is smooth, circular. -1 or :: mm. in diameter, and radiately grooved in correspondence 

 with the internal complete mesenteries. In retraction it is depressed for some distance within 

 the ealice, assuming a cup shape; bul sometimes the peristomial region is elevated. The 



