10 PERCY SLADEN TRUST EXPEDITION 



the polyp, viz. in the spaces formed by the dissepiments. This could not be the case il 

 the whole of the polyp actually moved up the corallite, and often the dissepiments are not 

 complete partitions stretching right across the calyx. In the face of these facts, it seems 

 to me that the dissepiments are partitions periodically formed across portions of the 

 corallite and of the polyp. When a full series of such horizontal partitions is formed 

 between the septa, etc., the polyp would be divided into two parts, the lower part bemg 

 cut oflF from that above ; the lower part degenerates while the polyp soon recovers its 

 normal height. Of course where a dissepiment is to be formed the body-wall has to be 

 folded horizontally inwards, and between the two layers of calicoblastic ectoderm thus 

 formed the calcareous material is secreted. By the successive formation of dissepiments 

 in this way the corallites grow in height, with the living polyps occupying only the 

 uppermost chambers. A further reason for holding the view suggested here is the 

 fact that synapticulee are actually formed as horizontal rods between septa piercing 

 through the intervening mesenteries and a dissepiment is nothing more than a 

 flattened-out synapticulum partitioning off an interseptal space'"'. Indeed, every stage 

 between a rod-like synapticulum and a complete dissepiment can be seen in the dry 

 specimens. 



Ogilvie (108, p. 157) referred to a similar origin of the dissepiments in the following 

 words : " The aboral wall draws itself up gradually during the period of active deposit at 

 the septal edge, forming during its updrawal the arched development. Finally the period 

 of ' pause ' sets in, and the body-wall between the septa remains supported on its many- 

 arched floor, which is then completed and thickened." 



Nematocysts. Boulenger (10) used the term nema,tocyst for " the actual stinging 

 capsule " and nematoblast for " the cell in which the former is embedded, and of which 

 the cnidocil and the stalk are part." In this paper I use the term nematocyst to mean 

 the whole apparatus, viz., the sac, thread and basal filament (if present), the reason for 

 which will become evident from the account of their probable development. 



I am inclined to agree with Hadzi and Boulenger that nematocysts are ectodermal 

 structures ; their occasional presence in the endoderm of certain polyps may be due either 

 to subsequent displacement or to migration. 



Three main types of nematocysts have been met with in the polyps studied, which 

 I refer to as types I, II and III ; all of these are liable to variations in the different 

 species. R. Hertwig (67) recognised, after a general survey of nematocysts in Actinians, 

 that their form and the nature of their thread was not the same everywhere, and 

 suggested that they might some day become of systematic importance. The subsequent 

 work of van Beneden (8) on Cerianthides and of Carlgren (20 and 21) on Actinians 



* It is doubtful if there is any real difference between true and pseudosynapticula. The former 

 according to Pratz and Ogilvie are said to possess centres of calcification, whereas the latter are described as 

 formed by the union of septal granulations, and hence devoid of any such centres. The Astrseidae are said to 

 have only such pseudosynapticula, but in some of my microscopic sections of their coralla distinct dark centres 

 are visible. Indeed, if the hard parts are formed in the way Bourne has shown, i.e. outside the calicoblastic 

 layer of ectoderm, and not by a calcification of its "cells," the dark centres and lines will have none of the 

 significance that Ogilvie attached to them. Costal synapticula are seen in sections of species of Favia and 

 Goniastrea. 



