BRITISH SPONGES 



121 



ones, through which the water is expelled, around the margin. 

 When the sponge is out of the water and inactive, the two 

 opposite sides of the bag are practically in contact, but, when 

 active, the cavity is filled with 

 water by means of the whip- 

 cells that line it, and the sides 

 of the sponge are then more 

 or less convex. 



The ciliated sycon (Sycon 

 ciliatum), fig. 70, though of 

 a very different appearance 

 externally, is similar in 

 structure to Grantia. It is 

 also found in similar situa- 

 tions, and is not uncommon 

 on many parts of the South 

 Coast, from Wey mouth 

 westwards. The other exam- 

 ple, Leucosolenia botryoides, 

 shown in fig. 71, is a branch- 

 ing calcareous sponge, con- 

 sisting of a number of tubes, 

 all united to form one com- FIG. 70. Sycon ciliatum 



mon cavity which is lined 

 throughout with whip-cells. It is usually found attached to weeds. 



Nearly all our British sponges belong to the group Demo- 

 spongia common sponges ; but the members of this group present 



FIG. 71. Leucosolenia botryoides, WITH PORTION MAGNIFIED 



a great variety of form and structure. Most of them have a 

 skeleton consisting of siliceous spicules, but some have a horny 

 skeleton, somewhat after the nature of that of the toilet sponges ; 

 and others, again, have fleshy bodies entirely, or almost entirely, 



