3O Invertebrata. 



attached below (fig. 17), and having a single wide open- 

 ing above, the osculum. The outer wall is a continuous 

 protoplasm with many nuclei, the inner is built up of 

 separate cells, each bearing a single long cilium sur- 

 rounded by a collar-like ridge. The pores traverse 

 these two layers, and when the outer wall thickens 

 they lengthen into canals. These canals may be 

 uniformly lined by flagellate cells, or the ciliated cells 

 may be restricted to dilated areas of the passages. In 

 the wall and around each of the pores are needle- 

 like spicules of carbonate of lime, usually united in 

 threes, and arranged in a radiated manner like the 

 three legs of the Manx coat of arms. Sometimes 

 they are in pairs or in fours. 



When a cluster of these simple vase-like sponges 

 grows closely pressed together, the outer walls of the 

 several animals partly coalesce with each other ; the 

 pores of each animal communicating with the inter- 

 spaces, which then often appear branched. Thus a 

 complex canal system grows up, according to the 

 degree of thickening of the wall and the amount of 

 coalescence of the separate elements of the clusters. 



In a living sponge, currents of fluid set in through 

 the minute pores on the surface, setting out in large 

 streams through the osculum. These currents are 

 kept up by the waving of the flagella which bedeck 

 the protoplasm masses that line the canals, and as 

 these currents traverse the canals the small organic 

 particles which they carry in are taken up by the cells 

 of the wall in the same manner as food particles are 

 swallowed by Rhizopods. Thus each cell nourishes 

 itself directly from the surrounding water. 



