316 G. p. BIDDER 



was found of Text -fig. 12) there is a constant current down 

 the slope away from the shore. The hypothesis is possible that 

 the fan-shaped form only occurs in response to the stimulus 

 of a constant current (compare Note 2) across which its plane 

 is extended ; but that if the current turns tidally from all 

 points of the compass, the fan grows up across each direction 

 in turn, so that an open cup is formed. 



The advantage of the fan in still water is shown in the text, 

 but in a current turning tidall}?- the efferent stream will be 

 driven back on the sponge for half the day. In such a position 

 the vertical oscular stream is the common form, because this 

 forms an equal angle with the supply from whatever point it 

 comes. Oyster-shaped sponges, with oscula on the edges, 

 are possibly from a channel where the tide runs alternately 

 from two opposite points ; they may be called ' pectinate '. 



Sponges living always in the surf, or long flexible sponges, 

 such as C h a 1 i n a o c u 1 a t a , which point downstream from 

 their stalks, have of course no need to do more than to lift the 

 outflowing w^ater sufficiently from their surface for the current 

 down wdiich they lie to bear it free of their more apical 

 parts. 



The conditions discussed in this paper affect sponges which 

 are left long in tide-pools, and sponges which inhabit depths 

 where there is inappreciable wave-motion, and where currents 

 are feeble. 



Note 5. Calculations of Pressure and Optimum Size 

 of osculum. 



(Mathematical basis of the paper.) 

 The loss of energy in a tube from resistance due to viscosity 

 in unit of time is 



^TTIJill-h, 



where u = velocity, 



b = length of lube, 

 [J. = index of viscosity. 



