22 HYDRACTINIID^. 



(fig. I, c). The appearance of the framework itself as seen 

 in section is that of a series of tubes laid side by side 011 

 a plate of chitine, and closely appressed one to the other. 



The tubular orifices are completely filled in with coenosarc. 

 Above they rise into many spinous projections so as to 

 exhibit a jagged outline ; below they rest uniformly on the 

 chitinous base. Here and there smaller spinous processes 

 are given off from the under surface of the latter, and pene- 

 trate the mucous layer that invests it. 



The structure of the framework seems to be of this 

 kind. From a thin basal lamina of chitine rise numerous 

 chitinous lamellae, terminating above in serrulated edges, 

 which sometimes run parallel to one another and some- 

 times anastomose. The spaces between them form the 

 channels in which the soft cosnosarcal stolons are contained; 

 and from these rise the polypites and the spiral and ten- 

 tacular appendages. The passages or tubes thus formed 

 are covered in above, not by a solid wall, but by a chitinous 

 network, which stretches across them a little below the 

 free serrated edges of the lamellae. 



Through the meshes of this fenestrated covering the 

 fleshy matter passes and forms a superficial layer, filling 

 in the grooves between the ridges and overlying the frame- 

 work, with the exception of the points of the spinules. 



The larger spines owe their origin to the elevation of 

 the tubes at certain points. 



The chitinous crust of Hydractinia, then, is in no true 

 sense a " foot- secretion ; " it is a modification of the ordi- 

 nary stolonic base of the hydroid zoophyte, and is secreted 

 in essentially the same way. The peculiarity consists in the 

 reticulated covering of the chitiuous tubes, allowing of the 

 outgrowth of the coenosarc and the consequent formation 

 of a naked superficial layer. 



The mode of development will be understood by a 



