126 ZOOLOGY FOR MEDICAL STUDENTS chap. 



is usually complicated, small rounded' chambers containing choanocytes 

 communicating with two sets of tubular channels the one inhalent the 

 other exhalent. 



The spicules are composed of silica and are either tetraxon — with 

 four rays diverging from a point at equal angles — or monaxon. The 



Fig. 6o. 

 Skeleton 'of Euplectella. 



latter are frequently united into a continuous framework — not however 

 by actual fusion between the siliceous substance of the spicules as is the 

 case with the Hexactinellids — but by the interposition of a peculiar 

 cement substance allied to silk in its chemical composition and known 

 as spongine. In various members of the group an interesting modifica- 

 tion of the skeleton has come about by the increase in amount of the 

 cement substance and a corresponding reduction of the siliceous spicules 



