40 THE ANTHOZOA 



is thickened to form a mesenterial filament ; in complete mesenteries 

 the filament commences at the stomodaeum, and ends at a short 

 distance from the insertion of the mesentery on the basal disc ; in 

 incomplete mesenteries the filament commences some little way 

 below the insertion of the mesentery on the peristome, and ends 

 below in a similar manner. In the upper and lower parts of their 

 courses the mesenterial filaments are straight, but their middle 

 portions are thrown into a number of coils, the mesentery itself 

 being plaited in a corresponding manner. The structure and 

 histology of a filament differs in different parts of its course. In 

 the upper part of its length the filament is trefoil-shaped in section 

 and has the structure shown in Fig. XIX. 5. The central lobe is 

 the cnido- glandular tract (Nesseldriisenstreif of German authors), 

 the lateral lobes are the ciliated tracts (Flimmerstreifen). In the 

 middle of the filament the cnido-glandular lobe disappears, the two 

 ciliated. tracts remaining; and in the lower portion of the filament 

 the ciliated tracts disappear, the median cnido-glandular lobe re- 

 appearing and forming the whole of the filament. Acontia are 

 filamentous offsets from the lower edge of the mesentery, having 

 the same general histological structure as mesenterial filaments. 

 They are characteristic of the family Sagartidae. 



The gonads are borne on the mesenteries, forming band-like 

 thickenings on that part of each mesentery which lies internal to the 

 longitudinal retractor muscles and below the level of the stomodaeum. 

 Actinia eguina is dioecious, as are many other Actinians, but some 

 members of the group appear to be monoecious. 



The radial chambers into which the coelenteiun is divided by 

 the mesenteries communicate with one another, not only by way 

 of the axial space into which they all open, but also by perforations 

 in the mesenteries themselves ; these are mesenterial stomata. In 

 Actinia the stomata are found in the uppermost inner angles of 

 the complete mesenteries, close beneath the mouth, and are 

 probably the result of incomplete union of the mesentery with 

 the stomodaeum. They are known as internal stomata. 



In some other Actiniae, e.g. Tealia crassicornis and Adinoloba 

 dianthus, external stomata are present. These are circular openings 

 situated in the upper third of each mesentery, nearer to the body 

 wall than to the peristome, but separated by a space from both. 

 Those genera which have external stomata also possess a strong 

 circular muscle band which runs right round the body just beneath 

 and outside of the outermost circlet of tentacles. This muscle 

 band, consisting of an axis of mesogloea thrown into folds along 

 which muscle ^ibres are arranged, projects into the coelenteron, 

 and is attached to the body wall by a thin sheet of tissue. It is 

 known as Rotteken's muscle. 



In Actinia the coelenteron communicates with the exterior by 



