THE ANATOMY OF ALCYONIUM DIGITATUM. 385 



mesogloea closed over it. It will be noticed that the long dorsal mesenterial 

 filaments are all on one side, i. e. the axial (Marshall) of the coelentera. The 

 spicules in the colony are much more numerous at the periphery than in the 

 deeper parts of the colony. The coelentera of neighbouring polyps do not 

 actually run into one another, but they are in communication with one another 

 by means of canals. 



Fig. 8. — Diagram of a section through a portion of a colony to show that 

 the secondary polyp cavities (ii) do not run into the primaries (i), but are in 

 communication with them by means of canals. The peripheral canals, spicules, 

 and other details are omitted. 



Fig. 9. — The disc of a polyp in the second stage of retraction. The ten- 

 tacles are contracted, but have not yet begun to bend over. 



Fig. 10. — The disc of a polyp in the first stage of retraction. This drawing 

 was made from a preserved and stained preparation of an expanded polyp, and 

 shows the arrangement of the muscles (»«.) on the disc, and the spicules of the 

 peristome and of two of the tentacles. These muscles are all ectodermic. 



Fig. 11. — A drawing of the disc of a living and fully expanded polyp, showing 

 the bullate base of the tentacles and the shape of the mouth. 



PLATE 38. 



Fig. 12. — Transverse section through an expanded polyp ( x 130) in the 

 region of the stomodseum, showing the folds in the epithelium lining the stomo- 

 dseum (not present in the expanded polyp of A. pal ma turn), the siphono- 

 glyphe, the arrangement of the muscles on the mesenteries. &c. 



Fig. 13. — Vertical section through a retracted polyp. On the left-hand 

 side the section passes through the side of a tentacle, and through one of the 

 long dorsal mesenterial filaments ; on the right-hand side it passes through the 

 centre of the tentacle and the siphonoglyphe. The stomodseum is seen to be 

 folded on itself. 



Fig. 14. — Vertical section through the extensible portion of a polyp ( x 70), 

 showing the mesentery bearing the transverse protractor muscles, the longi- 

 tudinal retractor muscles, one of the ventral mesenterial filaments, and the 

 siphonoglyphe. 



Fig. 15. — Transverse section through one of the pinnae of a tentacle, show- 

 ing the ectoderm with its nematocysts and the lumen in the endoderm. 



Fig. 16. — Transverse section through one of the dorsal mesenterial fila- 

 ments. 



Fig. 17. — Vertical section through the surface of a decalcified colony of 

 Aicyonium, showing the ectoderm with its covering of slime, the mesogloea 

 containing the endodermal canals, the empty spaces which contained the 



