CLASSIFICATION. 89 



Group. DlCTYONINA. 



The skeletal-spicules are normal hexactinellids, whose rays are fused together 

 by a common siliceous envelope so as to produce a firmly-united meshwork, which, 

 when regular, has cubical interspaces. The spicular nodes or centres may be 

 either simple or octahedral. A dermal layer of modified six-rayed spicules usually 

 present. Flesh-spicules may be present or absent. 



Family 1. — Euretid^e, Zitt. 



Sponges cup-shaped, cylindrical, turbinate, laminate or branching. Skeletal 

 mesh regular; the spicular nodes simple. The dermal layer consists of a 

 thickening of the exterior layer of the skeletal mesh, occasionally also a delicate 

 meshwork extends completely over the surface, and covers the apertures in the 

 sponge-wall. The structure of the root-appendage similar to that of the body of 

 the Sponge. 



Family 2. — Cosoinopobidj:, Zitt. 



Sponges cup-shaped, branching, frequently compressed, or with flange-like 

 walls extending from a centre. Very numerous, simple, straight, blind, radiate 

 canals, which open alternately on both sides of the sponge-wall. Skeletal mesh 

 close, and, owing to the numerous canals, irregular. Spicular nodes usually 

 simple, but occasionally octahedral. Dermal layer, when present, a cribriform 

 membrane. 



Family 3. — Mellitionidj:, Zitt. 



Sponges irregularly globular or branching. Walls completely perforated by 

 tubular canals, resembling the cells of honeycomb. Spicular nodes simple. Mesh- 

 work irregular. Dermal layer, a delicate siliceous meshwork which covers also the 

 canal-apertures. 



Family 4. — Callodiotyonid^;, Zitt. 



Sponges cup-, funnel-shaped, or compressed. Spicular mesh large and 

 regular ; spicular nodes octahedral. Dermal layer cribriform. No special canals 

 shown in skeleton. 



M 



