Part II. — Stichodacti/Unce and Zoanthece. 143 



As already ascertained by M°Murrich, there is no special concentration of the 

 endodermal muscle to form a sphincter ; indeed, the musculature, if anything, 

 becomes more feeble towards the apex. 



The verrucae are readily distinguished from the rest of the column by the 

 absence of gland cells from the ectoderm, and by the nuclear zone being broader 

 and staining more deeply. 



Sections through the acrorhagi reveal a thinner wall, an absence of gland 

 cells, and small nematocysts. 



In the tentacles, both the ectodermal and endodermal muscles are well- 

 developed on mesoglceal plaitings. The nematocysts in the former layer are 

 extremely small, and, in both, the fibrillar layer extends for some distance from 

 the mesoglcea. Zooxanthellaa are abundant in the inner layer. Where the 

 sections include one of the tentacular swellings, the enlargement is seen to be due 

 to a slight increase in thickness of the endoderm, but more especially of the 

 mesoglcea. 



The disc is much like the tentacles in structure, but thinner-walled, and fewer 

 nematocysts occur in the ectoderm. The mesoglcea is thrown into elongated 

 branching folds to serve as an additional support to the endodermal circular 

 muscle. 



Sections through the wart-like projections reveal slight, hollow upgrowths of 

 the disc ; the mesoglcea and musculature become so attenuated as to be with 

 difficulty recognizable, and the ectoderm is thinner than elsewhere, but ap- 

 parently does not exhibit any new structural elements. Like M c Murrich, I 

 have been unable to determine if the tubercles are actually perforated, but the 

 delicacy attained by both the mesoglcea and ectoderm at the apex is an indication 

 that the production of a temporary opening by any pressure from within would be 

 a matter of very little difficulty. It has already been noted that water may be 

 emitted in the living condition. M c Murrich (1889, pi. iv., fig. 11) gives a 

 figure of a section through one of the disc tentacles. 



The stomodaaum is very elongate and oval-shaped in transverse sections, and 

 extends almost across the ccelenteron, the pair of directives at each extremity 

 being much shorter than the lateral complete mesenteries. In longitudinal 

 sections, the stomodseum is comparatively short. Its walls are thin, and the 

 ectoderm is thrown into irregular vertical folds, partly followed by the mesoglcea. 

 The gonidial grooves are clearly indicated at each end, their walls unfolded, 

 and not much thicker than the rest of the stomodseum. At each end the groove 

 is prolonged for some distance below the lateral walls, the directives remaining 

 attached all the way. All the other mesenteries, however, are very deeply 

 concave along their free edge towards their attachment to the stoniodasal wall, so 

 that in transverse sections through this region a short portion of each mesentery 



T2 



