Part II. — Stichodactijlince and Zoanthece. 185 



musculature. Distally the mesenterial endoderm closely resembles that of the 

 column-wall, but contains a greater number of the glandular cells, with highly 

 refractive contents. Proximally it becomes swollen, and contains many granular 

 particles of various sizes, while elongated gland-cells are still more numerous. 



The parieto- basilar muscles are developed for some distance along each face of 

 the mesentery, and are continued a short way on to the column- wall. There is 

 no trace of any mesogloeal folding or pennon. 



Both cycles of i^erfect mesenteries remain connected as far as the inner ter- 

 mination of the stomodiEal-wall. I have not been able to determine any definite 

 order in which they become free, but the directives at one end remain united 

 further than the opposite pair, the laterals being the first to cease their connexion. 



The imperfect mesenteries project for some distance within the coelenteron. 

 In section they are almost as broad as the complete mesenteries, and the mesogloea 

 terminates in numerous processes, each surrounded by a muscular layer, which, 

 so far as it extends, is as strongly developed as on the first cycle. All the 

 endocoelic and exocoelic spaces are practically equal, and the mesenteries by no 

 means fill the coelentcric space. 



The terminal edge of the stomodseum is reflected as a whole, so that in sections 

 through this region the wall is double in all its three layers (PL xiii., fig. 4), 

 The reflected ectoderm passes for some little distance outwardly along each face 

 of the mesenteries, and aj^pears in perfect continuity with the tissues forming the 

 mesenterial filaments. At first the filaments are very irregular and narrow in 

 outline, forming only a slightly rounded termination to the mesenteries ; they are, 

 however, histologically very distinct from the rest of the mesenterial epithelium. 

 Lower they become more characteristic, and are either rounded or cordate in 

 section (PI. xv., fig. 3). 



The mesenterial filaments are remarkable in that only the middle terminal 

 lobe, the glandular streak or Nesseldriisenstreif, is ever developed ; the lateral 

 lobes, bearing the ciliated streak or Flimmerstreifen, so characteristic of most 

 Actiniaria, are never produced. 



A little below the aboral termination of the stomodseum the mesenterial 

 endoderm is considerably swollen immediately behind the filament, so as to produce 

 in section somewhat the appearance of a trilobed filament ; but these enlargements 

 cannot be regarded as at all comparable with the lateral lobes of the more usual 

 Actinian filament. Except in length of the constituent cells they differ in no 

 important respect from the remainder of the mesenterial epithelium into which 

 they often graduate insensibly, while the distinctly lobed character is not presented 

 by all the mesenteries. Further, the mesogloeal axis never sends a branch into 

 these swellings for the support of the cells, as is the case where the Flimmerstreifen 

 are developed. Such a filament is characteristic of the Madreporaria, and 



