386 SYDNEY J. HIOKSON. 



spicules before they were dissolved away iu acid, aud the spicule-formiug 

 cells {sp. c). 



Fig. 18. — Transverse section through a dorsal meseuterial filament and a 

 portion of the mesentery. The groove iu this case lies freely open instead ol 

 being nearly closed, as it is in iig. 16. 



Fig. 19. — Transverse section through one of the ventral mesenterial fila- 

 ments showing the gland-cells {g.c.) and flagellate cells (/-c). 



Fig. 20. — Transverse section of an expanded polyp (x 130) below the level 

 of the stomodajum, to show in section the six endodermic mesenterial fila- 

 ments (Vmf.) and the two ectodermic mesenterial filaments (Vm/.). 



Fig. 21. — Transverse section through a portion of a mesentery to show the 

 great retractor muscle-fibres (R. msc.) on their branched supports of mesogloea 

 (coloured darker in the figure), aud the very delicate protractor muscles (/?. 

 msc.) on the opposite face of the mesentery. 



Fig. 22. — Longitudinal section of a tentacle in a partially retracted condi- 

 tion such as that shown above in Fig. 9. The ectoderm (ecL) is shown con- 

 taining several nematocysts, and the ectodermic nerve plexus {n.p.). 



Fig. 23. — A small piece of the endoderm taken from the polyp tube a little 

 way below the termination of the ventral mesenterial filaments to show the 

 intercellular spaces. 



Fig. 24. — The same taken considerably lower down, showing that the inter- 

 cellular spaces are much larger and the endoderm-cells much more shrivelled 

 in appearance. 



Fig. 25. — A small portion of the endoderm covering a gonad, showing that 

 iu that region no intercellular spaces can be seen. 



PLATE 39. 



Fig. 26. — A section of a portion of the mesogloea and endoderm of a polyp 

 cavity taken about an inch below the surface of the colony, cut with the free 

 hand from the living animal, stained in weak methyl blue, and subsequently 

 treated with pyrolignic acid. In the mesogloea there may be seen a number 

 of very small stellate cells connected with one another by fine branching pro- 

 toplasmic processes. In addition to these there are a number of spherical, 

 spindle-shaped, or elongated cells collected together into strings or clumps, 

 and connected at intervals with the plexus of fibres from the stellate cells. 

 The plexus of fibrils may be seen to be connected with a plexus iu the lower 

 layer of the endoderm. 



Fig. 27. — A string of cells in the mesogloea from another preparation. 



Fig. 28. — A number of cells obtained by scraping the walls of a polyp tube 

 and treating with osmic acid and picro-carmine. a., k., l.,o., aud «. are all tyj)ical 



