Morphology of the Madreporaria. 101 



or another of their establishment. The three conditions are 

 represented in figs. 4-6, from camera lucida drawings of actual 

 sections. 



The earliest stage obtained is that represented in fig. 4. 

 Twelve complete mesenteries are present and eight incom- 

 plete members, all attached to the inner boundary-wall of the 

 polyp. From the disposition of the musculature and the 

 relation of the complete to the incomplete members there is 

 no difficuhy in ascertaining wliich are the primary twelve 



Diagrammatic arrangement of the mesenteries in a polyp of Pontes, 

 showing the manner of increase of the mesenteries beyond the six 

 primary pairs. These Litter are indicated by the thicker radial 

 lines, and numbered /- VI. Four new bilateral pairs, A-D, have 

 been successively added at one region within the entocoele of the 

 ventral directives, ///, III, the latter having become pushed far 

 apart (cp. fig. 6). In some polyps of Porites the new mesenteries are 

 added wuthin the dorsal directive entocoele instead of the ventral. 



mesenteries. They are indicated by the same numerals as in 

 the simple polyp. Ventrally they have become widely sepa- 

 rated from the dorso-ventral plane which passes througli the 

 entocoele of both pairs of directives, but in a symmetrical 

 manner, so that six are on one side and the corresponding six 

 on the other. As yet additional mesenteries are found only 

 on the ventral (axial) side of the polyp, situated within the 

 directive entocoele [HI, 111). They consist of two bilateral 

 pairs of complete mesenteries and two incomplete pairs {A~D)^ 

 the lettering not necessarily possessing any significance as to 

 the order in which the mesenteries appeared. Each member 

 of the new outermost complete pair {A) forms with the 



