Morphology of the Madreporaria. 149 



side. The groups are separated by continuous sepia, one 

 within the entocoele of each of the six primary pairs of 

 mesenteries. 



The above is tlie condition of nearly all the polyps on any 

 colony of S. hyades^ but occasionally examples are found in 

 which the mesenteries have undergone an increase. Such a 

 polyp is represented in fig. 5 h. Compared with the previous 

 figure, a pair of complete and of incomplete mesenteries have 

 been added on the left side of the directive axis, and a single 

 pair of incomplete mesenteries on the right side. As before, 

 the additional members conform in size, and relationship with 

 the stomodKum, with the mesenterial pairs already present. 

 Such a simple stage of increase, before a third pair of direc- 

 tives has arisen, serves to demonstrate that new pairs of 

 mesenteries may appear on botli sides of the directive axis. 



Otlicr enlarged polyps of S. hjades have been examined 

 by means of fections, but they merely repeat what has been 

 already established by examination of Cladocora and StephanO' 

 coenia, as to the manner of increase of the mesenteries. The 

 arrangement of the septa also supports the conclusions gained 

 from the mesenteries. 



Figs. 6 a-c represent the septal plan of three distinct 

 corallites of S. hi/ades. Fig. 6 a shows the number and 

 arrangement of the septa in nearly all the corallites consti- 

 tuting a colony. Twenty-four septa occur, arranged in six 

 groups of three, with an alternating continuous septum be- 

 tween any two groups. The condition corresponds with the 

 section of the polyp in lig. 5 a. Fig. G b represents the septa 

 of one of the few enlarged calices. In this thirty-six septa 

 occur in nine groups, and present exactly the same arrange- 

 ment as in the normal corallites with twenty-four septa. 

 With the increase in the number of septa, no additional 

 cycles are developed, thus differing from the plan generally 

 followed in corals. Fig. 6 c shows a calice with forty-three 

 septa, nearly double the number characteristic of most coral- 

 litts. Here the tricyclic character of the septa is still retained, 

 and fission is evidently in progress. When the process is com- 

 pleted, and the remaining five septa necessary to complete 

 the grouping in threes are formed, eacli moiety will exactly 

 resemble one of the ordinary calices. No corallite of S. hjades 

 has been found in which more than forty-eight septa 

 occur, thus showing that the septal increase is not continued 

 indefinitely, but only until the normal number of septa is 

 doubled. 



The details, as to the mesenterial arrangement, occurring in 

 the enlarged and bioral polyps of Oculina diffusa, are so 



