IV] CLASSIFICATION 91 



Order 2. Heterocoela. 



Sponges in which the choanocytes are restricted to special 

 chambers which may be cylindrical as in Grantia or spherical 

 as in Leucandra. 



Class II. HEXACTINELLIDAE. 



Sponges in which the skeleton consists of a coherent network of 

 siliceous spicules each consisting of three axes placed at right angles 

 to one another. The flagellated chambers are large and cylindrical 

 but are separated from the central space by a system of canals. 

 The central space may be deep and narrow and covered with a plate 

 pierced by numerous oscula, or short, open and shallow. 



These sponges inhabit as a rule very deep water and most species 

 are provided with a tuft of long needle-like spicules which root them 

 in the soft mud which forms the bottom of the sea at these depths. 

 It is a most interesting fact that the flints which form regular rows 

 in our English chalk have been proved in many cases to contain the 

 remains of Hexactinellid sponges. As the chalk is a deposit on the sea- 

 bottom similar to the globigerina ooze on the floor of the Atlantic 

 one would expect it to have been richly sown with these sponges. 

 Class III. DEMOSPONGIAE. 



These sponges derive their name from the fact that their spicules, 

 which are always siliceous, are arranged in cords so as to form a net- 

 work traversing the substance of the sponge. The spicules composing 

 these cords are nearly always cemented together by a horny elastic 

 material called s p o n g i n. The flagellated chambers are always extreme- 

 ly small and there is never a central chamber. Besides the skeletal 

 spicules, as those composing the cords are called, smaller ones called 

 .flesh spicules are scattered singly in the intervals of the network. 



There are several exceptional genera in which interesting modi- 

 fications occur. 



Oscarella is totally devoid of any skeleton and has the appearance 

 of a whitish yellow scum on the rocks to which it adheres. Euspongia 

 possess spongin cords but these cords have no spicules in them, and 

 for this reason it can be employed for domestic purposes. 



Two fresh-water species, namely, Spongilla lacustris with a bush 

 like appearance and Ephydatia fluviatilis with an encrusting form, 

 are often found growing on the side of canals and on the timbers of 

 river-locks or weirs in Great Britain. The two species are bright 

 green when they grow in the light, but they are pale flesh-colour 

 when they grow in the shade. In Canada similar species adhere to 

 stones in the river St Lawrence. . 





