fRANSACTIONS OF SECTION t). 687 



The island of Pemba possesses a barrier of reefs and limestone Islands off its 

 west coast, inside which are long bays which penetrate into the heart of the 

 island, and were famous in the days of the slave trade as secure hiding-places for 

 Arab dhows from our cruisers' boats. Along the mainland coast is a succession of 

 reefs and islets, forming a continuation of the barrier sj-stem of which the island 

 of Zanzibar is a swollen portion. The channel portion between it and the main- 

 land, though much narrower than that of Zanzibar, is like it in its depth of from 

 15 to 30 fathoms. At Chale Point, and again to the north of Mombasa, this 

 barrier becomes curiously regular and narrow, almost like an artificial breakwater. 

 But here, as elsewhere, coral is quite absent, the reef being formed of elevated 

 limestone. The conclusions reached are summarised as follows : — ■ 



1. There are here no reefs due to growth of corals and nullipores in situ. 



2. There have been, in geologically recent times, great growing reefs, the 

 upheaval and crystallisation of which have formed the rock of the whole coast 

 and outlying islands and reefs. 



3. That all the forms characteristic of growing coral reefs have been carved 

 out of this upheaved limestone by the eroding and solvent action of the sea. 



Examples : Fringing reefs, east coasts of Zanzibar and Pemba. Barriers, off 

 the mainland and west coast of Pemba. Atolls, Mnemba and certain reefs from 

 Zanzibar Channel and elsewhere. 



2^otes on the Coral Reefs of the Indian Ocean. 

 By J. Stanley Gardiner, If. A. 



4. Septal Sequence in the Coral Siderastrma. 

 By J. E. DuERDEN, Ph.D., A.R.C.Sc. {Lond.) 



The six members of the first cycle appear simultaneously within the entocoelea 

 of the first cycle of mesenteries. Shortly after six septa appear in a dorso-ventral 

 manner within the six primary exocoeles ; later they become bifurcated peripherally, 

 either by direct extension of the original septum or by the production of separate 

 nodules, which afterwards fuse. 



The second cycle of mesenteries having appeared, new septa arise peripherally 

 within their entocceles in the same radii as the six primary exosepta. Later 

 these second-cycle entosepta fuse with the original primary exosepta, and become 

 the secondary septa of the mature corallite, while the bifurcations of the six 

 primary exosepta now form the third cycle of twelve exosepta. The exosepta of 

 the third cycle afterwards bifurcate, and on their appeai'ance the third-cycle 

 mesenteries are included by them. New septa then arise within the entocceles of 

 the third-cycle mesenteries, fuse with the third-cycle exosepta in the same radii, 

 and constitute the adult third-cycle septa. The twenty-four exosepta of the 

 fourth cycle are formed from the bifurcations of the temporary third-cycle septa. 



Exosepta are thus present at each cyclic stage in the growth of the corallite, 

 alternating in position and corresponding iu number with the sum of the entosepta. 

 They never become entosepta, but always constitute the outermost cycle of shorter 

 septa; only the entosepta have any ordinal significance. The developmental 

 relationships between the entosepta and exosepta are closely comparable with 

 those first established by Lacaze-Duthiers for the entotentacles and exotentacles 

 of actinians. With the exception of the first, the members of each septal cycle 

 follow a dorso-ventral succession, display a bilateral symmetry for some time, and 

 ultimately assume an approximate radial plan. 



