Morphology of the Madreporaria. 389 



at the opposite extremities of the system, wliere growth is 

 generally proceeding more rapidly than elsewhere. No new 

 directives are found ever to arise. A variable number of 

 mesenterial pairs is associated with each stomodasum, the 

 hexameral plan being wholly departed from. Except in 

 one doubtful case, the plane of fission has been found to be 

 always included within the entocoele of two opposite pairs of 

 complete mesenteries. 



During the subsequent growth of the multioral polyp the 

 original linear arrangement is soon lost, owing to the fact that 

 one lateral area may grow more rapidly than another, and so 

 give rise to the sinuous outlines of the mature colony. The 

 variability in the rate of growth of different regions would 

 seem to be the main cause of the irregularity of the cyclical 

 plan. The smallest mesenteries become larger and ultimately 

 reach the stomoda^um, while others arise to constitute the 

 lower orders. It is doubtful, however, as to how far mesen- 

 terial orders or cycles should be recognized in fissiparous 

 polyps, seeing that the members of a lower order assume 

 later the position and relationships of the higher orders. 

 New mesenttries are intercalated among the others wherever 

 growth is proceeding more rapidly, and this without reference 

 to any cyclic plan. With the increase in the number of 

 mesenteries additional stoniodtea arise at somewhat regular 

 intervals, but not, however, in such a manner as to constitute 

 new individual polyps. Over a score of colonies of Manicina 

 in various stages of growth have been sectionized ; yet in no 

 instance has more than the two primary pairs of directives 

 occurred. Examination of parts of the mature colonies has 

 also failed to reveal such. It must therefore be assumed that 

 in any colony of Manicina^ however large, only two possible 

 pairs of directives occur, situated at what are to be regarded 

 as the two morphological extremities of the colony. 



Results similar to the abuve have been obtained from the 

 early stages in the develo[)nient and fission of Favii fragum, 

 though examples are not so readily obtained as in the case of 

 Manicina. The simple unioral polyps are found to be 

 regularly hexameral, and in two bioral polyps studied 

 fission had been introduced during the course of the formation 

 of the third cycle of mesenteries. As in Manicina, the 

 fission-plane in Favia is practically at right angles to the 

 elongated directive plane, and is included within two cnto- 

 ccelic chambers belonging to opposite sides ; the two pairs of 

 directive mesenteries also remain at the opposite extremities. 

 Likewise the number of mesenterial pairs associated with 

 each stomodseum is variable. 



Ann. & Mag. N. Hist. Scr. 7. Vol x. 29 



