300 



G. P. BIDDER 



works of engineers, absolute magnitudes are determined by the 

 relations of the consequences of increase in length to the con- 

 sequences respectively of increases in area and in volume. 



But sponges are not mere coalesced Protista : they have 

 varied cellular differentiation and at least two organs. The 

 first is the perforated membrane of tissue which is formed by 

 the flagellate cells, or on which they stand (Text-fig. 8), thereby 

 ensuring to them absolute separation of the water which leaves 



L \ t f r I o r 

 Wall of L e u c a n d r a a s p e r a. 



them from the water which supplies them ; this organ is 

 possessed by no choanoflagellate, and characterizes the sponges. 

 The second is the canal system, which we may better call the 

 hydraulic organ, in which the form of every part is wonderfully 

 modified for the advantage of the whole aggregate. In the 

 canal system all agree that progressive changes have occurred 

 again and again in similar order along many lines of descent 

 in sponges. We can show these changes to have as one of their 

 necessary consequences large increase in oscular velocity, with 

 consequent increase in the diameter of supply, and that there- 

 fore each successive change has left the sponge a more self- 



