THE ANATOMY OP THE MADREPOEARIA. 253 



What Moseley has termed "the contorted mesenterial filaments," 

 a mass of coils lying on the side of the mesenteries, appear to me, after 

 careful investigation to be, in part, at least, organs corresponding to 

 the acontia of Actiniae, namely, long lamellar offsets of the free edge of 

 the mesentery, with one edge thickened to correspond to the mesen- 

 terial filament, and charged with very large nematocysts. They pro- 

 trude in some instances, as above stated, through definite openings in 

 the mouth-discs. Their exact origin from and relation to the mesen- 

 teries I have not been able to detect, owing to the brittle condition of 

 the specimens, which did not allow of their being dissected out. 



The ova are developed on all three orders of mesenteries. As their 

 origin and position do not appear to differ from the type described 

 by the brothers Hertwig for Actinia, no figures are given. I have not 

 seen the testes, hence Flabellum may be regarded as dioecious. The 

 filament is present along the whole course of the free edge of the 

 mesentery, including that region in which ova are developed. The 

 latter is mostly below the part which is characterised by great con- 

 tortion of the free edge and by (T) the giving off of acontia. 



iii. Histology. — The ectoderm of the mouth-disc (fig. 6) is charac- 

 terised by deeply-staining, very numerous nuclei ; and has distinctly 

 the appearance of a secreting layer. It probably produces a similar 

 secretion to the slime poured forth in quantities by an irritated 

 Actinia. 



This figure (which is a section along the line a, fig. 2) is taken from 

 a well-grown polyp, and shows traces of the originally basal ectoderm 

 which secretes the corallum (the calycoblasts of v. Heider) (ch., fig. 6). 

 In a younger and actively-growing polyp these are much more 

 definitely marked (ch , fig. 7). The nuclei lie in a gelatinous-looking 

 matrix, which stains slightly with borax carmine, but in which no cell 

 outlines are distinguishable. In the calycoblast layer surrounding the 

 septum, at the same height and in the same polyp, the nuclei are 

 much rarer (ch., fig. 8). 



The characters of the ectoderm alter considerably on the tentacles ; 

 as above mentioned, it is on them raised into a series of knobs, each 

 of which is a " battery " of nematocysts. A transverse section through 

 the wall of a tentacle is shoAvn in fig. 9, and exhibits the structure of 

 a battery ; the nematocysts are confined to the peripheral part, and 

 behind them lie a very large number of nuclei, probably instrumental, 

 as was first suggested by v. Heider, in the formation of the cells which 



