FORM OF A SPONGE 313 



may be of use to naturalists who are able to observe in a large 

 tank the length of the jet from a mollusc or other fixed organism, 

 and the width of the aperture. It is thus possible, for jets 

 not exceeding 180 cm., to obtain a value for the approximate 

 issuing velocity, and therefore of the quantity of water filtered 

 by the animal and of the effective work performed by its 

 cilia. The correction for temperature is theoretical. 



Thus the figure of 8-5 + 1-5 cm. per second, which I have 

 finally adopted as the mean oscular velocity, at 18° C, of 

 Leucandra asp era gigantea (Text-fig. 1), healthy and 

 in natural conditions, is deduced from the oscular streams of 

 30 cm. and even 45 cm. long, measured from recently gathered 

 sponges. These indicate a mean oscular velocity of 8 cm. per 

 second, and from the reduction which this undergoes under the 

 best aquarium conditions, it may be concluded that the velocity 

 of any gathered sponge is less than in the sea.-'- 



The formula was deduced from measurements of velocity 

 on sponges some time in the tanks whose streams had shortened 

 to 15 cm., 10 cm., 5 cm., or even 1 cm. ; such languid velocities 

 being more easy to measure. The constant (12 + 2) has been 

 adopted as a compromise between a group of experiments by 

 different methods which agree on the value L /VB =10-5 ±1 and 

 several experiments which group themselves about L/F-B = 14." 

 The distance to which the cloud drifts will obviously be affected 

 by the position of the jet with reference to the walls of the tank : 

 with accurate physical experiment and definition of the condi- 

 tions a closer result could be obtained ; but for aquarium 

 observations it is probable that the conditions cannot be 

 sufQciently identical to reduce greatly the probable error of the 

 ratio L/FB. 



1 In cold w.ater, fiagella have a lower metabolism with which to drive 

 a more viscous fluid, and the energy of the oscular current is less. When 

 increase of temperature first becomes injurious, diminishmg vitality is 

 compensated by diminishing viscosity ; with still higher temperatures the 

 change in viscosity is less and the injury greater (cf. 'Linn. Soc, J.', 34, 

 p. 317). 



^ Experiments in June and July, temperature unfortunately not observed. 



