HALF AN HOUR WITH SPONGES. 57 



and in the darkest corners, certain patches of 

 red and orange. They are seldom or never more 

 than an inch in thickness, frequently nothing 

 like that, and exactly resemble Fi 2 o 



coloured lichens. Among the 

 species you are likely to come 

 across, we may mention two be- 

 longing to the genus Grantia eori- 

 acea and compressa. The former 

 is a bright vermilion-coloured 

 patch of soft but dense substance, 

 about an inch in diameter only. 

 The surface is covered by a series 

 of small orifices : the oscula, to 

 which we have referred. When 

 dead, this sponge turns to the 

 colour of a dried leaf, but when 

 alive it is covered by a layer of 

 firm sarcode, or "sponge-flesh," 

 which gives it a peculiar feel when 

 handled. Both these species of Mag. x 60. 

 Grantia possess spicules, most of which are three-rayed 

 (see Fig 21). Perhaps in the same cavern or hollow 

 you may find a larger species (Halichondria incrus- 

 tans), which, as its name implies, is incrusting the 

 rock with its thick, undulating surface, which is 

 slightly raised into pap-like ridges, and covered with 

 shallow, winding sinuses. On the top of each of these 

 ridges are the mouths, or " oscula." The whole sponge 

 may be several inches in diameter, and about three- 



