82 



J. E. DUERDEN. 



stages of septal growth secured by Lacaze-Duthiers in Astroidcs, 

 Flabelhim, Balanophyllia, Caryophyllia, etc., and also by \on 

 Koch in Astroidcs and Caryophyllia, the six primary septa ap- 

 peared simultaneously, and were equal from the beginning. 

 In Siderastrea the second cycle of six septa began to appear a 



few days after the primary cycle, 

 its members situated within the 

 exoccelic chambers, thus alternat- 

 ing with the six entosepta (Fig. 

 2). In a few polyps the septa 

 appeared simultaneously and were 

 all practically equal, but in most 

 individuals a marked difference 

 was manifest ; the dorsal and 

 middle pairs of exosepta arose 

 bilaterally in advance of the two 

 FIG. 2. Arrangement of the mesen- ventral pairs, and for a time the 

 teries and septa after the establishment dorsal pairs were a little larger 

 of the first and second septal cycles , 



T,, than the middle. Within a short 



six entosepta and six exosepta. The 



dorsal pair of exosepta are somewhat time the two ventral exosepta 



larger than the middle pair, and the appeared, but remained smaller 



middle pair are larger than the ventral , u , , . , -p, 



than the others. Thus at this 



pair, thus giving a bilateral character to 



the corallum. early stage a decided dorso-ven- 



trality in the development of the 



septa was apparent, which gave a bilateral character to the polyp 

 as a whole (Fig. 2). 



In most of the developing corals investigated by Lacaze- 

 Duthiers and von Koch the entoccelic and exocoelic cycles either 

 appeared together, or one shortly after the other, as in Sideras- 

 trea. In most cases it was found that the members of either 

 cycle arose simultaneously, in a truly radiate manner, without 

 passing through a bilateral stage, though Lacaze-Duthiers figures 

 a very decided bilateral condition in the early development of the 

 skeleton in Astroides (1873, PI- XIII., Fig. 29). 



The next stage in the growth of the septa in Siderastrea is 

 well defined, but is not so clear as to its significance. After the 

 condition shown in Fig. 2 was reached the septa began to enlarge 

 at their peripheral extremity; in some instances the enlargement 



