SPONGES 



Fi :. 88. 



Transverse section across an excurrent canal and surrounding choanospme of Cydonium 

 cosaster, Soil, e, excurrent canal ; /, flagellated chambers communicating with it by aphodal 

 canals ; i, an incurrent canal cut across ; s, a sterraster ; o, an oxea cut across. (Alter Sollas, 

 " Challenger " Report, X 125.) 



function, but also very frequently in morphological characters. Thus 

 certain forms of microsclere, such 

 as the commonly occurring asters, 

 conform to types of structure not 

 represented among the megas- 

 cleres. In this respect we find a 

 marked contrast with the Hexac- 

 tinellida, where all the spicules, 

 even the asters, are variations of 

 the one fundamental triaxon type. 

 Forms of the Spicules. In the 

 first place, a distinction must be 

 drawn between the simple (prim- 

 ary) spicules, on the one hand, 

 and the compound (secondary) 

 spicules or desmas, characteristic of 

 the sub -order Lithistida, on the 

 other hand. Since the desma is 

 itself founded, in most instances, 

 upon a primary spicule, we may 

 commence with the discussion of 



flip latrpr rent canal is shown on the left-hand side, 



near its commencement in the cortex. (After 



All primary spicules in the F. E. scimi/.e, 



