ANATOMY OF MUSSA AND EUPHYLLIA. 23 



sponding to the region that is covered externally by soft 

 tissues. Below this the corallum is invested by a distinct coat 

 of thin granular epitheca. The septa are united within the 

 calyx by dissepiments, thin obliquely placed lamellae of cal- 

 careous tissue placed at different heights in the interseptal 

 loculi, and meeting towards the centre of the calyx (see 

 fig. 4>d). 



The colour of my specimens and the tentacles differ from 

 Ehrenberg's account. The Mussa of Diego Garcia is of a dull 

 brown colour, with olive-green disc and tentacles, the latter 

 being numerous and moderately long. The polyps contract 

 energetically on being handled, and I was unable to preserve 

 any specimens with the tentacles expanded ; indeed, all my 

 specimens are so much contracted that the tentacles are no 

 longer recognisable even in section. A view of a system of 

 three polyps borne on a common stem is given in fig. 1 , the 

 drawing is taken from a specimen contracted in spirit, and is 

 half the natural size. 



Fig. 2 is a transverse section of a corallum of Mussa taken 

 just below the lip of the calyx. In each septum may be dis- 

 tinguished dark centres of calcification, around which are seen 

 a number of concentric, dark, and light lines of growth, mark- 

 ing successive additions of calcareous tissue. Evidence may 

 also be seen of centrifugal growth of each septum, due to the 

 deposition of calcareous matter on its peripheral extremity. 

 The septa are seen to be joined together by wing-like out- 

 growths, which fuse to form a theca. At a lower level the 

 theca would be seen to be consolidated by a continuation of 

 the peripheral lamellae of the septa over the intervening 

 bridges of tissue, which thus externally cement the conjoined 

 septa into a continuous whole. 



Anatomy of the Polyp. — A glance at fig. 1 will show that 

 the soft tissues of the polyp extend downwards for a consider- 

 able distance on the outside of the corallum. There is in fact 

 a well developed " Randplatte," as it is called by German 

 authors, and it contains extrathecal continuations of the 

 exocoeles and entocoeles separated from one another by peri- 



