Morphology of the Madreporaria. 151 



The mesenterial and septal conditions met with in the 

 enlarged polyps of Sieplianocoenia^ Solenas/rcca, and Oculimi 

 practically prove the same for tliese gemmiferous genera. In 

 all six genera, the addition of new mesenteries and septa, 

 beyond those characteristic of the species, does not lead to an 

 increase in the number of hexameral cycles or orders, but 

 merely to an increase in the number of mesenteries and 

 septa belonging to the original cycles. It may therefore be 

 assumed that this is characteristic of the enlarged polyps of 

 gemmiferous corals generally, and, further, that the products 

 of simple fission are truly bud-polyps. 



The products of simple fission, in species which are normally 

 gemmiferous, may be now contrasted with those found in 

 corals where fission is the usual method of vegetative growth, 

 and where budding seems to be unknown. In the third paper 

 of this series (1902'), 1 have given an account of the early 

 stages of fission in Manicina areolata (Linn.) and Favia 

 fragxim (Esper), and compared tiie results from these two 

 familiar corals with the characteristics of other species of 

 fissiparous corals, such as Mceandrina lahyrinthica (Ell. & 

 Sol.). It is there shown that the larval polyps o^ Manicina 

 and Favia develop according to the normal Hexactinian 

 plan for some time, and that the first fission-plane divides the 

 young polyp into practically equal halves, in an entoccBlic 

 plane at right angles to the directive plane or principal axis 

 of the polyp. The hexameral cyclical plan may or may not 

 be preserved in each moiety, but only one pair of directives 

 is present in each. ]\lanifestly, therefore, the products of 

 fission are from the beginning different from those in Madre- 

 porOf Forites, Cladocora, and others where two pairs of 

 directives occur. 



Fission continues in Manicina and its allies, and the 

 mesenteries become grouped in complete or incomplete 

 isocnemic pairs around numerous stomodgea, but soon lose all 

 their hexameral and most of their cyclical regularity ; and, 

 further, no additions are ever made to the two primary pairs 

 of directives. The products of fission in normally fissiparous 

 genera are not new polyps in the same sense as are bud- 

 polyps ; they are devoid of directives, and without any 

 hexameral cyclical regularity. Fission-polyps are more to 

 be regarded as fragments isolated from a complex large 

 polyp than as separate individuals. Morphologically, a sharp 

 line of distinction is established between polypal fission and 

 budding, though such is not generally recognized by students 

 concerned with the skeleton only. 



Polypal fission, as represented in the enlarged polyps of 



