COMPARATIVE ANATOMY OF SOME BRITISH ACTINIA. 539 



The spherical granules seen here are of the same kind as are found in 

 the ectoderm. They are more numerous near the free border of the 

 endoderm. Zooxanthellae are present in the endoderm of the column 

 wall, but are not very plentiful. 



There are 24 pairs of fully developed mesenteries, 2 pairs being direc- 

 tives. Alternating with these are the secondary mesenteries, so that 

 in all there are 24 primary pairs and 24 secondary pairs, making 48. 



The number of tentacles is the same as that of the mesenteries. All 

 the primary mesenteries except the directives are fertile, and in this 

 specimen bear gonads. The musculature in the mesenteries is strongly 

 developed, especially in the region near the column wall (Fig. 10). The 

 parieto-basilar muscle is cordate in shape with a stout projection on the 

 side towards the intramesenterial space. The mesogloea forms a short 

 stout stem where the muscle is attached to the column wall. The muscle 

 then broadens out rapidly, giving off short processes on either side, and 

 these are sometimes slightly branched. Most of them are narrow at the 

 base, and widen out into broad spherical knobs at the distal ends. 

 There are between 9 and 15 of these branches on either side. One side 

 of this muscle is continued into the large retractor muscle ; this is very 

 wide in the region near the column wall, but becomes narrower towards 

 the distal end of the mesentery, where it is continued into a very 

 narrow plate of tissue. The mesogloea continuing from the parieto- 

 basilar muscle becomes slightly narrower before it enters the retractor 

 muscle and is very thin where it enters the filament. On one side, 

 the mesogloea has short stout projections, these send out long and 

 delicate strands which are much branched. These strands are so numer- 

 ous that they are not easy to count ; the folds, including their chief 

 branches, number between 80 and 100. 



At the distal end of the muscle the folds radiate towards the middle 

 line. No mesogloeal processes are given off on the other side of the 

 muscle. The endoderm is continued from the column wall, along both 

 sides of the mesentery ; it is slightly narrower here and contains some 

 zooxanthellee. In some parts deeply stained glandular cells are 

 seen. 



The secondary mesenteries are very stout, but reach only a short 

 distance into the coelenteron. They consist chiefly of a parieto-basilar 

 muscle which appears to be almost fully developed. The retractor muscle 

 is beginning to be formed showing a feathery bunch of muscle folds. 

 In some cases these folds are pressed back owing to the development o£ 

 large gonads. 



