soUTAKY CORALS. 219 



morphologically equivalent to a true theca, the external portions of the exocoeles and 

 endocoeles are continued into the corresponding internal chambers, and when the 

 polyp is fully expanded the depth of these intermesenterial chambers must be 

 considerable, owing to the relatively great height of the exsert septa. Spirit specimens 

 are, of course, very much contracted, and the soft tissues are everywhere stretched 

 over and down in between the septa, pali and columella to an extent that makes the 

 interpretation of sections a matter of considerable difficulty. 



The Tentacles. These, as stated by Gardiner, are twenty-four in number, 

 one corresponding to each endocoele. Owing to contraction it is somewhat difficult to 

 determine the position of the tentacles in an expanded polyp, but it appears that the 

 twelve tertiary tentacles form an external circlet situated quite at the edge or even 

 outside the margin of the calyx. The six primary and six secondary tentacles form 

 an inner circle within the margin of the calyx ; they alternate with one another and 

 the primaries are nearer to, but still at some considerable distance from, the mouth. 

 The tentacles are thickly covered with knob-like batteries ot nematocysts, which are 

 conspicuous in sections. Each tentacle is attached at its base to the two members of 

 a mesenterial pair, and the longitudinal muscle fibres of the mesenteries are continued 

 up into and may be traced to the tip of the tentacle. In contraction the tentacles 

 are completely introverted by the action of the muscle fibres, and in spirit specimens, 

 owing to the excessive contraction of the mesenteries themselves, the invaginated 

 tentacles are doubled over the endoccelic septa and each appears in sections to be 

 prolonged downwards into two pockets, one in each mesentery of the pair to which it 

 belongs (fig. III.). 



The Peristome and Mouth . Owing to the distance of the tentacles from 

 the mouth, the peristome is of considerable extent. It is closely contracted against 

 the numerous pali and paliform lobes at the inner edges of the septa, and its contours 

 are scarcely distinguishable in spirit specimens. The mouth is an oval aperture 

 whose size varies very much in spirit specimens ; in some it is contracted to a 

 narrow oval, in others it is widely open, giving a clear view of the soft tissues 

 investing the columella. The mouth, according to all received ideas on Actinian 

 anatomy, should open into a stomodaeum, but in Heterocyatlius there is no definite 

 stomodaeum, in the sense of a longer or shorter tube lined by a modified epithelium. 

 The ectoderm of the peristome is very thin, and at the lips of the mouth it is some- 

 what thickened, and, as described below, there is evidence of a distinct sphincter oris 

 and a distinct dilator oris (radiating fibres) muscle in this region. Immediately 

 within the lips, but not uniting to form a conrplete tube, are the large gutter-shaped 

 " filaments " of the six primary mesenterial pairs which undoubtedly perform the 

 functions of a stomodaeum, and may be described as twelve discontinuous portions of 

 the stomodaeum, united only by the thickened but not modified ectoderm of the lips. 

 There is no trace of a sulcus or sulculus (gonidial grooves). The reduction of the 

 stomodaeum seems to be clearly due to the great development of the pali and the 



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