Morphology of the Madreporaria. 



145 



pairs, including ati additional pair of directives (D)," are 

 united with the stomodwum ; the corresponding additional 

 pairs of alternating second-cycle mesenteries also occur, and 

 a comparatively small number of third-cycle mesenteries. 

 The exact order followed in the appearance of the mesenterial 

 pairs beyond those present in tig. 2 a has not been established ; 

 tlie different poljps seem to show very diverse arrange- 

 ments. From the details available, it is not certain which are 

 the primary directives, and which the new pairs. It will be 

 observed, however, that all the new pairs arise as isocnemic 

 unilateral pairs, not as bilateral pairs, which is the manner 

 already shown to be characteristic oi Madrepora and Poriles. 

 The most important fact is that the additional mesenteries are 

 not arranged so as to add to the number of cycles origiiTally 

 present, as in the growth of coral and actinian polyps gene- 

 rally, but become constituents of the first, second, and third 

 cycles already established. 



Fig. 3. 



Transverse section through the sfomodaeal region of a bioral polyp of 

 C arluscvla having only a single system of tentacles and a single 

 column-w all. The y olyp is in pr.icess of fission in a median entocc3eIic 

 plane, nr.d when ccmplttcd the niesenleiial arrangement of each 

 polvp will closely resemble that of an ordinary bud-polyp like 



tig;2fl. 



A transverse section through the stomodteal region of an 

 enhuged polyp of Clodvcora, which has undergone partial 

 fission, is represented in fig. 3. In the living polyp, two oral 

 aperture? were present on a single disk, and the tentacles 



