92 SIDKRASTREA RADIANS. 



Four distinct stages in the development of the protosepta of Siderastrea 

 radians are thus recognizable : 



1. The simultaneous appearance of six equal entosepta, a few days after 

 the larva settles. A very narrow epitheca, distinct from the septa, appears 

 about the same time. 



2. The appearance several days later, either simultaneously or in a 

 dorso-ventral sequence, of six exosepta which are smaller than the entosepta 

 and alternate with them. The septa of both cycles are simple, spinous 

 wedge-shaped upgrowths from the basal plate. 



3. The appearance towards the periphery of most of the septa of one or 

 more short septum-like bars or skeletal nodules. 



4. The fusion of these detached fragments with the main septa, so as 

 to give rise to a broad peripheral termination which may be either simple 

 or bifurcated ; also the fusion of the exosepta with the entosepta by their 

 inner extremity. 



A distinct dorso-ventrality in the rate of growth is maintained from the 

 second stage onwards, particularly with regard to the exosepta, thereby giving 

 a bilateral symmetry to the calice. 



The protoseptal development of Siderastrea presents a general agree- 

 ment with that of other corals whose early history has been followed. 

 Lacaze-Duthiers (1873, 1897), however, found that in Astroides calycularis, 

 Balanophyllia regia, Leptopsammia, and Cladopsammia the six exosepta 

 appeared simultaneously with the six entosepta ; but in Caryophyllia cyathus 

 and others there is an interval between the two cycles as in Siderastrea^ 

 whereas in Manicina areolata, as I have shown (1902, p. 491), it appears 

 to be doubtful whether exosepta ever appear. Lacaze-Duthiers represents a 

 a decided bilateral condition in the early development of the skeleton in 

 Astroides (1873, plate xiv, fig. 29), but in other known cases the septa of 

 each cycle appear simultaneously, and are equal from the beginning. 



The simultaneous appearance of the members of one or both cycles of 

 protosepta and also of the prototentacles may be compared with the succes- 

 sive appearance of the pairs of protocnemes. The protosepta and prototen- 

 tacles resemble one another in that both appear a cycle at a time, and from 

 the beginning exhibit radial symmetry, whereas the protocnemes arise in 

 bilateral pairs according to a well-defined succession, and, for a time, display 

 a strong dorso-ventrality. The simultaneous appearance of all the members 

 of a cycle is maintained for the septa only so far as the first cycle or, at most, 

 the second cycle of the protosepta ; in the later growth of the organs a dorso- 

 ventral sequence is followed, quite as conspicuous as that of the mesenteries. 



