564 FOSSIL REMAINS OF SPONGES. 



then manifest great symmetry and method in their distribution, 

 with relation to the pores, canals, and vents. 



1231. The Geographical distribution of the Porifera is very 

 extensive; indeed it may be said to be almost universal. 

 Every coast, from the Equator to the highest Polar regions, 

 furnishes some kinds of Sponge ; but they exist in much greater 

 abundance in warm latitudes than in cold, and they attain also a 

 much greater size. They are all, of course, inhabitants of the 

 water only, and if long removed from it they lose their vitality ; 

 but there are many species which seem able to bear exposure to 

 the air between the intervals of the tide, appearing to flourish 

 equally well in deep water, or at a level which is occasionally 

 left dry. 



1232. There is some difficulty and uncertainty in regard to 

 the Fossil remains of Sponges; but it is probable that these 

 have come down to us from a very remote period of the earth's 

 history, and it may be reasonably supposed that Sponges were 

 among the earliest inhabitants of the ocean. These remains are 

 found in two states, Sometimes the whole tissue has been per- 

 meated by siiicious or calcareous matter ; so that, on the mass 

 being broken, its internal structure is very evident. In other 

 instances we have only the casts, which have been formed by the 

 subsequent filling-up, with stony matter, of the cavities left by 

 their decay. There is reason to believe that, in the Sponges of 

 ancient date, the siiicious spicula must have predominated ; for 

 we find their fossil remains almost always silicified, even in 

 calcareous rocks. Thus, in the Chalk (in which they greatly 

 abound) all the remains of Sponges present the character o flints. 

 Some of these flints, when broken, exhibit very beautifully the 

 structure of the Sponge ; and others possess only its external 

 form. Now, many of the Chalk fossils are infiltrated with car- 

 bonate of lime, and not with flint ; and this even when associated 

 with Sponges. In the same flint-nodules which envelope 

 Sponges, the shells of the Echinus ( 1092) are found converted 

 into crystallized carbonate of lime, and dense shells of Mollusca 



