286 I. IJIMA : HEXACTINELLIDA. I. 



are best developed in the plane of the primary branches bearing 

 them. The symmetry and regularity in form of the entire sponge 

 is however usually much disturbed by the bending and twisting 

 of the stem as well as by the not uniform development of the 

 lateral branches. 



In certain specimens, as an occasional occurrence a branch 

 may be swollen, either terminally or elsewhere, into an irregular 

 mass of considerable thickness. For instance, the specimen of 

 PL XII, fig. 1 shows such swellings in at least two places. 

 Cutting one open, I found no free cavity within but a space 

 traversed by loose parenchymalia, in which was imbedded the 

 body of a small animal — probably an Annelid. I think the 

 swellings are always mere abnormalities caused by certain extrinsic 

 objects. The process of their formation may in a measure be 

 compared to that by which all the branches arise in connection 

 with the commensal Hydrozoa and yet the latter comes near to 

 being intrinsic on account of its invariable presence. 



In some specimens it is not at all uncommon to observe a 

 thin web-like expansion of the spicular tissue at the axils of the 

 branches. One specimen which I have seen was jDarticularly 

 distinguished by the great abundance of the web-like plates not 

 only on the branches themselves but also between these and the 

 stem. On the other hand, a number of specimens nowhere 

 showed a similar development. 



Spicülation. 



The jKirenchymaUa are almost exclusively diactins of vari- 

 ous sizes. Very rarely among the accessor la there occur spicules 



