MATTHAI— RECENT COLONIAL ASTR^ID^ 19 



Ectodermal Musculature. This is extremely weak in all the polyps and consists of 

 (a) the filamentar processes occurring in the ectoderm of the oral-disc, edge-zone and 

 tentacles whose course may be traced from the nuclei to the mesoglsea, perhaps being 

 neuro-muscular offsets, and (6) a thin layer of fibres with a longitudinal or oblique 

 disposition, which has been recognised in the larger tentacles of some of the polyps. 

 The latter layer begins at the oral-disc and extends up along the entire height of the 

 tentacles, but does not appear to have any connection with the mesenterial musculature. 

 It perhaps assists in shortening the tentacles. 



In my polyps I have not found a circular endodermal or a longitudinal ectodermal 

 muscular layer in the column-wall nor are there ectodermal muscular fibres arranged 

 radially in the oral-disc as described by van Beneden for certain Cerianthidse (8, PI. 5, 



%• 7). 



Oral-disc (PI. 5, figs. 51, 52). The terms "oral-disc " (" disque buccal," van Beneden) 

 and " peristome"* have been loosely applied to the upper surface of the Anthozoon polyp. 

 In this paper the former name, which is synonymous with the "mouth-disc" ("Mundscheibe " 

 of German authors) of Fowler and Gardiner, is used for the circum-oral area delimited by 

 the outermost cycle of tentacles, this being conterminous with Rotteken's muscle, while 

 " peristome " is employed to denote the circum-oral space. The mouth is situated in the 

 centre of the oral-disc. 



The ectoderm of the oral-disc has a columnar facies, with oval nuclei of varying size 

 arranged along its middle. In these nuclei chromatin granules are visible, usually round 

 a somewhat larger darker spot, apparently the nucleolus. Above this layer of nuclei the 

 protoplasm is, as a rule, opaque, containing vacuoles usually of the raucous type, rarely of 

 the granular, while below the protoplasm is somewhat transparent and finely granular. 

 In many of the species deeper-stained protoplasmic areas are visible, each containing 

 a nucleus ; these diverging from the nuclei towards the free surface of the ectoderm. 

 Round nuclei are much fewer, more or less homogeneously stained, and occur in the lower 

 half of the ectoderm. Type I nematocysts (see p. 11) are present in varying numbers 

 in the upper half of the ectoderm, occasionally type II also. 



The endoderm varies in its thickness and in the extent of its vacuolization, the nuclei 

 being smaller than in the ectoderm and more or less homogeneously stained. Algae are 

 usually abundant, sometimes massed so that the endoderm is hardly distinguishable. 



When it is considered that the convolutions of the mesenterial filaments containing 

 nematocysts are frequently protruded through the oral-disc, that the peristomial ectoderm 

 is provided with nematocysts and mucous vacuoles, the latter secreting an adhesive fluid, 

 and that the tentacles which ai-ise from the oral-disc are charged with batteries of 

 nematocysts and also with mucous vacuoles, it is evident what an effective apparatus the 

 peristome is for capturing prey. 



* Faurot (42) takes objection to both these terms and introduces a new name " oro-tentacular disc " (disque 

 oro-tentaculaire), which according to him makes its appearance, in Hexactinians, at the same time as the first 

 eight tentacles. This name has no advantage over the older "oral-disc." Indeed, the latter is capable of wider 

 application, since in the coral genus Hydnophora — which I have recently begun to study — the tentacles are 

 arranged in circles round the conical eminences or monticules, without any reference to the mouth openings which 

 lie in the depressions between the monticules. 



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