3O Invertebrata. 



rooted by a flat protoplasmic expansion below (fig. 

 17), and have a single wide opening above, which 

 is named the osculum. Its walls are pierced by 

 numerous fine apertures or pores, which open di- 

 rectly into the central cavity. The wall consists of a 

 cluster of Monad-like cells, provided with a collar, 

 each sending out a pseudopod. In the wall and 

 around each of the lateral pores are needle-like 

 spicules of carbonate of lime usually united in threes, 

 and arranged in a radiated manner. Sometimes they 

 are in pairs or in twos. Others are like anchors, with 

 two flukes. 



As in most sponges the wall of the body-cavity 

 below each mouth is thick, not simple and membranous, 

 the pores elongate into canals. Most sponges also 

 grow in tufts or clusters arranged so close together 

 that the outer pores of the neighbouring, and closely 

 united animals communicate with each other ; thus a 

 complex canal system grows up, according to the 

 degree of thickening of the wall and coalescence of 

 separate elements of the clusters, as well as by the 

 superaddition of interspaces, which are often branched, 

 between the separate individuals or elements. 



In a living sponge, currents of fluid set in through 

 the minute pores on the surface, setting out in large 

 streams through the oscula ; thus there are many 

 mouths and few outlets. These currents are kept up 

 by the waving of the flagella which bedeck the pro- 

 toplasm masses that line the canals and cover the 

 skeleton, 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. 



