MORPHOLOGY OF THE MADREPORARIA. 



8l 



corals, but their results throw no certain light upon the difficult 

 problem of the manner of increase of the septa beyond the two 

 primary cycles, nor of the relation of these to the later cycles. 



Observations which I have been able to make upon the growth 

 of the septa in larval polyps of the common West Indian coral, 

 Siderastrca radians (Pallas), show the unreliability of assuming 

 the developmental sequence from adult relationship, and give an 

 interpretation to the later septal sequence altogether different 

 from any hitherto proposed. 



Larvae of Siderastrca, fixed to fragments of glass, and capable 

 of being examined as transparent microscopic objects, were fol- 

 lowed in the course of their development as young polyps for a 

 period of four months, and the order of appearance of their septa 

 determined as far as the completion of the first three cycles. 



In every case it was found 

 that the six members composing 

 the first cycle appeared simultane- 

 ously. This takes place shortly 

 after the larva settles, at a stage 

 when the young polyp has six 

 pairs of mesenteries, arranged as 

 in Fig. I, where all the mesen- 

 teries are complete except the fifth 

 and sixth developmental pairs. 

 The septa are alike in size and 



Situated at equal distances apart FIG. I. Diagrammatic 'arrangement 



within the entocoeles of the six of the mesenteries and septa on the 



c appearance of the first cycle of six 



primary pairs of mesenteries. _. 



septa. Ihe septa are situated within 



Later development proves, as the six prirnary entocoeles, and the 



would be expected, that the six six alternating mesenterial chambers, 



primary septa of the mature coral- devoid of se P ta > are the P rimar >' ex " 



occeles. The directives are situated 

 lite are the direct enlarged rep- at the opposite extremities . the two 



resentatives of the six septa first bilateral pairs of incomplete mesenteries 

 to appear in the larval polyp. are the fifth and sixth P airs in the 



mesenterial sequence. The upper 



As regards the primary cycle border is regarded as donal and ^ 

 in Siderastrca the surmise that lower as ventral, 

 adult size corresponds with de- 

 velopmental sequence is therefore correct. Moreover, in the early 



