198 Proceedings of the 



lum. On this plate, by a further process of secretion from 

 the lower ectoderm, the grooved spines are erected. That 

 the upper layer of the ectoderm is naked over the greater 

 part of its surface, or only covered by a thin epidermis ; but 

 occasionally this layer also takes its share in the secretion of 

 the corallum, and in that event produces the smooth, conical 

 spines, the concavity of which it fills. 



14. I have been unable to detect the existence of true cells 

 either in the endoderm or ectoderm of the polypary. These 

 membranes appear to consist essentially of structureless modi- 

 fications of elemental tissue, more or less vacuolated, similar 

 to that we find in the protozoa, and to which the term " sar- 

 code " has been applied by Dujardin. Accordingly, we find 

 that the finger-like processes given off at the borders of the 

 polypary constantly flow into each other when they meet, 

 like the prolongations of sarcode projected from the pores of 

 the Rhizopoda, or the outer layer of Actinophrys. 



15. The walls of the tubes which permeate the polypary are 

 frequently loaded with the coarse granular matter of a brown- 

 yellow or crimson colour, which is found in the endodermic 

 tissue of all hydroid polyps. This matter has been considered 

 by some authors as a glandular secretion of the nature of bile. 

 It appears to me to be identical with the brown matter which 

 exists in the bodies of many of the Protozoa, such as Vorticella 

 campanulata, and which in other species of the same genus is 

 replaced by a substance having all the properties of chloro- 

 phyll. In the Hydra viridis, also, we find green globules pos- 

 sessing the chemical reactions of chlorophyll substituted for 

 brown matter in the endoderm. Whatever purpose this granu- 

 lar matter may serve in the economy of the hydroidse, it al- 

 ways occurs in excess in situations where the vital functions 

 are most actively carried on ; such as the tips of growing sto- 

 lons, the alimentary tubes of the polyps, and the ovaries and 

 spermatic sacs. 



16. The Ectoderm of the Polypary contains great numbers 

 of the highly refractive capsular bodies to which the terms 

 " tricho-cysts," " thread-cells," or " stinging organs," have 

 been applied. They are of two kinds (fig. 11), differing very 

 distinctly in size ; the larger exceeding the smaller ones by 



