60 SIDERASTREA RADIANS. 



all respects resembled a distinct colony. Siderastrea radians thus exhibits 

 the phenomenon of a coral reaching its colonial stage by direct union or 

 aggregation of a number of independent larvae, the larvae being at first free 

 swimming and wholly unconnected with one another. The close association 

 is probably to be understood as a thigmotactic phenomenon, and calls for a 

 more extended study with abundant material. The subject of " Aggregated 

 Colonies in Madreporarian Corals " has been more fully discussed in " The 

 American Naturalist," June, 1902. It is there shown that similar associa- 

 tions are at times met with in other corals. 



Externally the opaque larvae seemed all alike, but sections reveal slightly 

 different stages of growth as regards the number of mesenteries and their 

 connection with the stoniodaeum. The sections further demonstrate that the 



FIG. 5. Aggregation of fifteen polyps derived from the same number of originally free-swimming larvae. 

 Only the basal skeletal appearance is represented. 



opacity is due to the thickness of the endoderm and the numerous Zooxan- 

 thellse within its cells. Furthermore, the gastro-ccelomic cavity is yet 

 unformed, or very limited in extent, the larvae being a nearly solid mass of 

 cells. The distended larvse, on the other hand, already showed from the 

 outside different stages in the development of the mesenteries, and the 

 internal cavity was more fully established (cf. plate i, figs. 3, 4). 



After fixation all the larvse became distended, the walls more trans- 

 parent, and mesenterial divisions, twelve in number, were rendered conspicuous. 

 From the beginning of fixed life some few were more advanced than others. 

 These were the individuals born in an already transparent state. On set- 

 tling they were nearly as large again as the other larvae, and their develop- 

 ment throughout was more rapid. 



A few abnormal larvae were extruded and swam about like the rest. 

 The oral extremity of these was double and the aboral single, the bifurcation 

 taking place about the middle of the length (plate i, fig. 2). Lacaze-Duthiers 

 (1873, p. 312) likewise found such double monsters to be somewhat frequent 



