iv PORIFERA 55 



are calcareous, but they are four-rayed, or consist of needle-like 

 structures pointed at both ends. 



Si/con dliatum is very common round our coasts; it forms 

 little whitish oval sacs about an inch high, with a ring of 

 silvery spicules round the osculum. 



Sycon, Clathrina5 and Grantia all belong to the class 

 Calcarea, which includes all those sponges in which the 

 spicules are formed of carbonate of lime. 

 Six-rayed or Certain sponges are known as Six-rayed or 



Glassy glassy sponges, because the spicules are siliceous 

 Sponges. i ns tead of being calcareous, and they are typically 

 six-rayed, though modifications of the six-rayed type may occur. 

 These are all deep-sea forms, and many of them have a very 

 beautiful symmetrical structure, such as is seen in the well- 

 known Venus' Flower-basket. In these forms, as in Sycon, 

 the body-cavity is comparatively simple, with a single central 

 cavity and thimble -shaped lateral extensions of it in the 

 body-wall. 



In Common Sponges (Demospongia), the struc- 

 ture * s mucn more complicated, owing to the 

 fact that the flagellate radial chambers, which 

 extend from the central cavity, have become narrower and 

 branched ; also, in many, the flagellate cells have become con- 

 fined to special little enlargements or chambers in the radial 

 canals. 



Further complication is introduced by the original simple 

 body having become much branched, and all the branches 

 having fused together, so that there may be a great many 

 oscula scattered over the surface of the sponge as well as the 

 very numerous smaller inhalent pores. 



In most of these complex sponges the skeletal part consists 



of siliceous spicules, with one or four rays. These may occur 



alone or combined with a network of horny or silky threads, 



formed of a substance known as spongin. In 



Sponges COTnmon Bath Sponges the spongin alone is present, 



forming the tough supporting skeleton which we 



buy as a toilet sponge. 



In the best cup-shaped Turkey sponges, these fibres are 

 specially soft and fine, and the pores are so small, that we 

 do not find in them the sand and shells which so often get 

 lodged inside the larger, coarser bath sponges. 



