Royal Physical Society. 197 



pary; and the latter is clearly seen to be permeated by a 

 beautiful system of anastomosing canals (fig. 12) connected 

 with the hollow bodies of the polyps. Within these canals 

 may be detected an intermittent flow of fluid, containing par- 

 ticles, the dancing motion of which indicates the presence of 

 ciliary action, and which, having passed in one direction for 

 a short time, are arrested, and, after a slight period of oscilla- 

 tion, commence to flow in an opposite direction. 



12. A few branches of one of the tubular stolons was sub- 

 mitted to a microscopic power of 600 diameters. They were 

 found to be composed (fig. 4), as is the polypary of every 

 hydroid zoophyte, of an external and internal membrane, — the 

 " ectoderm " (c), and " endoderm " (d) of Allman, — inclosed in 

 a wide tube or corallum of transparent chitine (b). At the 

 growing extremities of the branches the corallum was absent, 

 and its place supplied by an epidermis (a), a soft, glutinous 

 layer closely investing the ectoderm.* Further down the 

 branch the corallum appeared, secreted by the ectoderm be- 

 neath the epidermis, by which last substance it continued 

 coated, and from which it derived its adhesive property. A 

 delicate chitinous investment may also be detected on the 

 creeping tubular fibres from which the stolons of Hydractinia 

 take their rise ; but I have not satisfied myself as to its 

 presence on the entire upper surface of adult polyparies. In 

 specimens of Hydractinia, however, growing in the shell, we 

 find that the spines of the corallum, although varying in 

 shape and structure, may be classed in two divisions; the one, 

 solid and deeply grooved, and clothed by the polypary (fig. 2) ; 

 the other, smooth, conical, and hollow, and inclosing a process 

 of the polypary in their interior (fig. 3). 



13. From the above observations, I conclude that the poly- 

 pary of this zoophyte consists of a single layer of endoderm, 

 inclosed between two layers of ectoderm. That the lower 

 ectodermic layer, as it grows over the shell, attaches itself by 

 its " colletoderm," and secretes the horny plate of the corai- 



* This layer which I have called the " colletoderm," (xoXX^ryu glutinator), 

 is constantly found covering the coralla of creeping zoophytes, where it fre- 

 quently forms a coat of considerable thickness, continued, also, as in Coryne, 

 over the body of the polyp. 



