Royal Physical Society. 195 



network, intersecting the surface in every direction. The 

 structure of many of the spines differs somewhat from that 

 above described ; some being hollow smooth cones, slightly 

 grooved or reticulated only at their base (fig. 3) ; while others 

 have their ridges pierced everywhere by foramina, and united 

 by irregular transverse bands, until they become a mere fenes- 

 trated structure of interlacing fibres. More rarely, we find 

 the spines here and there fused together in long serrated ridges, 

 running parallel with, or fitting into each other in a variety 

 of intricate patterns. Quatrefages has described the corallum 

 as an endoskeleton deposited in the substance of the polypary, 

 like the solid axis of Gorgonia. But I am satisfied that his 

 views on this point are incorrect, and that its mode of secre- 

 tion differs in no essential from that of the corallum of other 

 hydroid zoophytes. 



The Polypary. 



6. The polypary consists of a layer of semi-transparent 

 fleshy matter (either colourless, or tinted with light shades of 

 yellow, buff, or pink), which invests the corallum, and fills 

 up the grooves of its papillae, the interstices between its reti- 

 culations, and the cavities of its hollow spines. It is often 

 absent at the summits of the papillae, and is so thin over their 

 ridges, that the serratures appear through it as fine undu- 

 lating lines. 



7. The Polypary is the most important part of the zoophyte. 

 It secretes the corallum, renews it when injured, and extends 

 it along the shell. From it the polyps are developed ; and it 

 represents the trunk by which the whole assembly are united 

 to form a single plant-like animal. 



8. Quatrefages states, that the polypary is underlaid by a 

 network of tubular fibres, which pass between the polyps, and 

 unite their alimentary canals with each other. The existence 

 of this structure has been doubted by Dr Johnston, and alto- 

 gether denied by Van Beneden ; but I have repeatedly de- 

 tected the tubes permeating, rather than underlying, the poly- 

 pary, and have watched their development day by day, in 

 portions of that body growing over a transparent surface. 



9. In the less exposed parts of the shell the polypary fre- 

 quently passes beyond the papillary corallum as a thin mem- 



