310 



G. P. BIDDER 



system described by Vosmaer in Spirastrella).^ But 

 modifications concerned with the angle of supply demand 

 brief notice in regard to our main thesis. 



Stalks of greater or less length (Text-fig. 9) increase the 

 angle of supply in many sponges, and when this is the case 

 in sponges of any size, the osculum opens out, as admixture of 

 incoming and outgoing streams becomes less probable, and 

 oscular velocity therefore less important. This gives rise to the 



Text-fig. 10. 



^ 



Calyx lieberkuhnii (modified from 0. Schmidt). 



well-known Neptune's-cup form (Text-fig. 10), found in many 

 groups, and the expanded lip intervenes between the two 

 streams. 



If this cup becomes set on one side (Text-fig. 11), the efferent 

 stream passes away forwards, while the intake is at the back 

 of the cup, and the angle of supply approaches to 180°. In 

 still water, with the oscular jet horizontal, the length of the 

 jet becomes infinite, whatever its velocity ; because with the 

 angle of supply 180° there is no back eddy, and the friction 

 on the surrounding water serves by degrees to set it in motion 

 ^ Siboga-Expeditie, ' The Genus S p i r a s t r e 11 a ', p. 49. 



