ON THE STRUCTURE OF HYDRACTINIA ECHINATA. 85 



grooved spines. Even in these mature parts of the colony, 

 that part of the skeleton which connects the spinulcs and 

 spines together is often exceedingly thin and inconspicuous. 



The spinules are merely solid chitinous thickenings, irre- 

 gular in shape and size, projecting upwards from the chitin 

 which is attached to the whelk-shell. They occur all over the 

 skeleton, and are very conspicuous towards the thin edge of 

 the colony (PI. 1, fig. 1, a and b). 



The spines are one of the characteristic features of the 

 colony ; they are mostly deeply furrowed, having longitudinal 

 serrated ridges between the grooves. The ridges are con- 

 nected with one another by chitinous cross-bars, and fuse 

 together at the tip of the spine. Central chambers or a single 

 chamber communicate with the grooves. The whole spine is 

 covered by the general coenosarc, which is also continued into 

 the meshes of the framework, though it may often become 

 rubbed off from the tips of old spines where the shell has 

 come in contact with the ground. 



Allman (13) has figured and described a typical spine in 

 his account of H. ech inata. Sometimes the spines bifurcate 

 towards their apex, or are otherwise irregularly shaped. 



Between the spines and spinules the skeleton is in some 

 places deposited as a single thin layer, but it is usually secreted 

 in thin, irregular, reticulated layers, one above another. In 

 the older parts of the colony these layers, in vertical section, 

 have the appearance of chitinous strands varying in thickness 

 and running horizontally or obliquely (PI. 1, fig. 1, c), but towards 

 the thin growing edge of the colony they are more regular. 



At intervals throughout the skeleton there are spaces sur- 

 rounded on all sides by chitin, and containing degenerating 

 masses of coenosarc; such masses have become constricted off 

 from the rest of the coenosarc by unequal growth of the chitin, 

 the various stages of such constriction being demonstrable. 

 In some cases where the constriction and isolation are not com- 

 plete the degenerating coenosarc is seen to be in direct con- 

 tinuity with the coenosarc of the colony (PI. 1, fig. 2). In other 

 cases the spaces are empty. 



