64 THE STORY OF ANIMAL UFE 



exterior of the rounded mass of the bath-sponge 

 may be found numbers of such holes. We should 

 naturally suppose that these holes were the 

 mouths of the various sponge branches, espe- 

 cially since they lead to the central cavity of the 

 branch, and thus to that of the whole sponge; 

 and indeed they are known by the Latin name of 

 "oscula," little mouths. They are, however, 

 nothing of the sort ; the sponge once had a 

 mouth, a single one, when it was young, but the 

 adult sponge has lost it. For the young sponge 

 is at first a little free-swimming, two-layered ani- 

 mal of the type which has been described above 

 as the gastrula larva. When it gets old enough 

 to settle down in life, it sinks upon some suitable 

 surface, and becomes fixed to it, mouth down- 

 ward: the mouth is thus lost. How, then, is the 

 animal to be fed ? As it grows, there is developed 

 in its substance a system of hollow spaces, which 

 communicate with the exterior by means of mi- 

 croscopic pores. Through the latter, water is 

 drawn in, and passes, after devious wanderings, 

 to the central cavity of the animal, whence it is 

 expelled by the so-called osculum. At first, the 

 young sponge has but one cavity and one oscu- 

 lum; but by degrees the sponge branches and 

 spreads, the cavity of each new portion remain- 

 ing in connection with the main cavity. If, as 

 they grow in size, the branches touch one an- 

 other, they sometimes coalesce a fact which 

 renders the growth of the sponge in some cases a 

 very complicated matter. 



It will be seen from the above description 

 that the sponge is a sort of living filter. As the 

 water passes in through the pores, it deposits in 

 the substance of the sponge all the little or- 



