620 Haddon and Shackleton — A Revision of the British Actinice. 



In looking at a side view of a mesentery such as that of Z. macgillivrayi (PI. lxiv., 

 fig. 5) it will be seen that the ectoderm of the oesophagus passes continuously on 

 to the mesentery where it suddenly becomes greatly thickened, and is thrown into 

 transverse folds ; the whole thickening has a crescentic form, first coming upwards 

 and then downwards losing itself in the mesenterial filament. The ectoderm is 

 reflected on both sides of every one of the perfect mesenteries (PI. lx., fig. 6), 

 and presents a very characteristic appearance in transverse sections (PL lxiv., 

 figs. 4 and 6). The folds often present a pinnate appearance, but they are rarely 

 accurately symmetrical on each side. 



In some species the endoderm is implicated in the upward reflection of the 

 lower edge of the cesophagus ; this is especially noticeable in Parazoanthus axinellce 

 (PL LX., fig. 6), but it is not a feature of any morphological importance. 



As above mentioned the reflected ectoderm passes gradually into the mesen- 

 terial filament. The characteristic trefoil (P. axinellce) (PL lx., fig. 6) or V-shape 

 {Z. macgillivrayi) (PL lxiv., fig. 7) of the latter in transverse sections is continuous 

 with the peripheral pair of folds of the reflected ectoderm. 



The lateral elements of the upper portion of the craspedum gradually become 

 shorter, so that eventually only the median portion is left (PL lx., fig. 7). This 

 transition takes place very shortly below the lower level of the oesophagus. 



Nematocysts are numerous in the simple lower portion of the mesenterial 

 filament ; but they are not readily seen in the upper portion, and we have not 

 observed any in the reflected ectoderm. Unicellular glands and pigment granules 

 also occur in the filament. 



The length of the craspeda varies even in the same genus, it apparently being 

 dependent upon the height of the polyp ; for example in Z. jukesii, which is a 

 short species, the filaments extend nearly down to the commencement of the 

 coenenchyme, whereas in Z. macgillivrayi they cease about half way down the 

 polyp. 



Mesoglcea. — The mesogloea of the mesenteries exhibits a certain amount of 

 variation in thickness ; for example, in the oesophageal region of Parazoanthus 

 dixoni (PL lx., fig. 9), the mesogloea is relatively thick, whereas in Epizoanthus 

 paguriphilus (PL lx,, fig. 5) it is quite thin. 



Canals. — In describing the canals in the mesogloea of the body-wall, we have 

 already alluded to the fact that in the three species of the genus Zoanthus from 

 Torres Straits which we have examined, they not only arise from the ectoderm, 

 but pass into the mesenteries. 



Hertwig (1882, p. 115) describes in Z. dance (?) {of. 1888, p. 36) a basal 

 " septal canal" "in the supporting lamella of the septa in immediate proximity 

 to the wall." He "never could make out any connexion between this septal 

 canal and the ectodermal cords [i. e. canal systemj of the wall " ; and he is 



