FORM OF A SPONGE 



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Olynthij each ejecting an efferent stream 1 mm. wide, and raise 

 up their colonial wall to enclose a common efferent aperture 

 1 cm. wide, thus forming a hypothetical Ehagon with an efferent 

 aperture having its area equal to the aggregate oscular area 

 of the 100 Olynthi. We can visualize the stream from each 

 Olynthus as a thread, and the stream from the Ehagon as a cord 

 of a hundred threads issuing side by side. We see at once 



Text-fig. 6. 



Efferent streams. Hypothetical Rhagon with wide aperture. 



that most of the threads lie altogether inside the cord, with 

 their surfaces entirely protected by other moving threads from 

 friction against still water. The mass and initial velocity of 

 the Rhagon's jet is that of its 100 constituent jets ; but for 

 each centimetre of length the external surface of the Ehagon's 

 jet is only one-tenth of the total surface of the constituent jets, 

 consequently the friction to which it is exposed is one-tenth 

 of their friction, and the combined jet will travel about ten 

 times as far as each of the constituent jets would go separately. 

 Coalescence is therefore advantageous in increasing the 

 diameter of supply ; and, as in all living things and in the 



