306 GILBERT C. BOURNE. 



but differing from this in histological detail. The more com- 

 plicated and effective acontia of Sagartia are probably de- 

 veloped from a simpler form, such as this. 



In Fuugia the mesogloea appears to break up dendritically 

 in the swelling of the acontium, instead of ending in a T-shaped 

 swelling, as in other forms {vide fig. 12) . 



The acontia of the tertiary and succeeding mesenteries lie 

 coiled up in the exocoeles at the bases of those mesenteries, and 

 in section appear to fill up the greater part of those spaces 

 {vide figs. 13 and 15). 



Since the septa are large compared with the interseptal 

 loculi, and since the septa are always in the entocoeles, it fol- 

 lows that the two mesenteries forming a pair are pushed apart 

 from one another, whilst the adjacent mesenteries of contiguous 

 pairs lie close together. So much is this the case, that when 

 a decalcified animal is cut across transversely the adjacent me- 

 senteries of contiguous pairs appear from their position to form 

 pairs, and in old specimens they may become fused together at 

 a little distance from the periphery, as is shown in fig. 11. 



The Coelenteron. — The coelenteron is represented by the 

 axial space lying below the stomodaeum, the peripheral cham- 

 bers known as exocoeles and endocoeles, and the extra-thecal 

 space lying on the aboral surface between the theca and the 

 external body wall. 



The axial portion of the coelenteron is not definitely circum- 

 scribed. Above, it opens freely into the stomodaeal invagina- 

 tion ; below, it is limited by the investments of the trabecular 

 columella ; at the sides it is partially limited by the thickened 

 borders of the mesenteries above described. So far as it can 

 be considered as a definite cavity it is no doubt the cavity in 

 which digestion is chiefly eflfected, the process being carried 

 out by the secretions of the gland-cells of the mesenterial 

 filaments. 



The relations of the peripheral parts of the coelenteron are 

 difiicult to understand in this, as in all other Madreporarian 

 corals, but are further complicated in Fungia by the presence 

 of synapticula. The coelenteron is composed of all those 



