20 Coelenterata. 



at a depth of about 160 fms. and on this the banks severally arose. This 

 plateau is roughly 300 miles long and 65 miles broad, it is sharply defined, 

 sloping off evenly, in 2-4 miles outside the limits of the reefs, to over 1000 fms. 

 It probably represents an island or a line of peaks (left after the great de- 

 pression of land in the Tertiary period) by the erosion and denudation of which 

 the plateau was formed. Regular reef-building corals do not live below 30 fms. 

 in any such luxuriance as would be requisite if a reef were to be built up. 

 Reef corals feed mainly or almost entirely by means of their commensal 

 algse (dead organic matter was found in only 12 % of the polyps of Pocillo- 

 pora and Astrcsa}. Their limit in depth depends on the penetrability of sun- 

 light through sea water. Probably the light is not of sufficient intensity for 

 organisms to actually live below 75 fms. (at about which depth Lithothamnion 

 is found in the E. Indies). From this level upwards there is a group of or- 

 ganisms (including Goniopora, Alveopora, Dendrophyllia, Heliopora, Mill., and 

 many solitary Madreporaria) which becomes more and more dominant, reaching 

 its maximum at about 45 fms. but continuing to exist up to 30 or 25 fms., 

 at which depth, in the tropics, reef corals obtain full sway over all other 

 sedentary forms of life. These latter feed by their commensal algse and, as 

 the depth decreases and the light becomes more powerful they obtain the upper 

 hand at about 25 fms. and only a few of the deeper organisms (notably Mill. 

 and Hel.) manage to maintain their position. Of genera of corals at all im- 

 portant on surface reefs in any region only Madrcpora, Seriatopora and Po~ 

 cillopora were represented below 30 fms. ; reef-corals procured from beyond 

 this depth show clearly in their growth that the increased depth is deleterious 

 to them. The author obtained 40 coral colonies from an area which had been 

 absolutely cleared less than 3 years before and calculated from their amount 

 of growth in this time that the reefs might grow in height at the rate of about 

 a fathom in 60 years. The author adds an account of boring (Sponges, Algae, 

 Lithodom-us, Sipunculids, Polychaeta especially Eunicidse, the Cirripede Lithotrya, 

 etc.) and sand-feeding (Holothurians , Echinids , Ptychodem, Sipunculids and 

 Polyclueta) organisms and points out their action and effects. 



Hickson( 2 ) finds in specimens of Amphihelia oculata and ramea a horny 

 membrane, similar in character and position to that found lining the tubular 

 axis of Neolielia porcellana. He believes (with Pratt, 1900) that these Madre- 

 pores secrete the membrane and that it is not (as has been suggested by others) 

 formed by some tubicolous worm which inhabited the hollow tube of the 

 corallum. 



Krempf describes a structure which occurs in 25 to 35 per cent of the indi- 

 viduals of Seriatopora, StylopJiora and Pcecilopora. In Ser. it is in the form 

 of a stout cord hanging from the roof of, and almost filling, the ventral direc- 

 tive chamber. It consists of hypertropMed finely granular endoderin cells sur- 

 rounding a mesogloeal axis, the latter being a prolongation of the mesogloea 

 of the oral disc. This structure first appears as an endodermic papilla in which 

 the elements soon become specialised and into which the axis of mesoglcea 

 soon penetrates. In Styl. the organ has similar relations but the mesogloeal 

 axis contains a cavity which ends blindly below but opens widely above to the 

 exterior and is lined with ectoderm continuous with that of the oral disc. In 

 Pcec. there are 3 of these organs (similar in structure to that of Ser.) in each 

 individual, the median one in the directive endoccel, the others in the neigh- 

 bouring chambers. These organs are invaginated tentacles which are greatly 

 modified in size (being, on an average, 16 times as large as a normal ten- 

 tacle) and in structure. 



