Ii6 PORIFERA OR SPONGES. 



or threads of sponge-stuff ; others become muscle-cells,, which 

 may be able to close the large exit pores ; others may contain 

 pigment, for the sponges are often brightly coloured ; others 

 become irregular binding cells, the beginning of what is called 

 connective tissue. Morever, some well-fed cells of this 

 mesogloea become ova, and others divide into balls of 

 spermatozoa. 



To avoid complications, we have not spoken of the cells 

 lining the canals which lead to and from the chambers 

 with collared ciliated cells. It seems that at least the more 

 external regions of the afferent canals are lined by an in-turn- 

 ing of the flat ectoderm cells, while the efferent canals at any 

 rate are endodermic. On these canals flat ciliated cells occur, 

 but they are very different from the monad-like collared cells 

 of the ciliated chambers. 



If you realise the effect of the budding and the fusion of 

 buds, the folding of the inner layer and the restriction of the 

 cilia to small chambers, the complication of the middle 

 stratum and the framework it builds, you will understand 

 how a complex sponge is derived from a simple cup. 



Kinds of Sponges — Perhaps the most convenient general 

 classification of sponges is that which distinguishes three 

 main sets : — 



(i) Calcareous (Calcispongiae), with spicules of carbonate 

 of lime ; 



(2) Siliceous (Silicispongiae), with spicules and threads of 



silica ; 



(3) Horny (Ceratospongise), with a framework of some- 



what horny material. 

 To these may be added a few forms which have no 

 skeleton (Myxospongiae). 



(i.) Calds^ongim. — The calcareous sponges have a world-wide distri- 

 bution in the sea, from between tide marks to'depths of 300-400 fathoms. 

 The simplest (Ascones) are simple cups such as we have described, and 

 lead by a gradual series of forms through Sycones, to a height of com- 

 plexity in Leucones. The purse-like Sycaruira (or Grantia) compressa, 

 is common on British shores. 



(2) Silicispongia. — The siUceous sponges are more numerous, diverse,, 

 and complicated. Very familiar is the marvellously beautiful skeleton 

 of .£«//afe//3— the Venus' flower-basket— a type of those whose (Hexac- 

 tinellid) spicules have six rays lying in three axes. Like the glass-rope 

 sponge {Hyalonema) and others, it lives anchored in the mud of the 

 deep sea. But let us rather mention a few of the commonest flinty 



