PHYSIOLOGY 235 



described, between an inhalant and an exlialant system of canals, 

 Avill necessarily produce a current passing inwards at the ostia 

 and outwards at the osculum. And the difficulty seems to be 

 increased when it is found ^ that the flagella in any one chamlier 

 do not vibrate in concert, but that each keeps its own time. 

 This, however, is of less consequence than might seem to be tlie 

 case. Two conditions are essential to produce the ol)served 

 results: (1) in order that the water should escape at the mouth 

 of the chamber there must be a pressure within the chamber 

 higher than that in the exhalant passages ; (2) in order that 

 water may enter the chamber there nmst be within it a pressure 

 less than that in the inhalant passages. But the pressure in 

 the inhalant and exhalant passages is presumably the same, at 

 any rate before the current is started, therefore there must be a 

 difference of pressure within the chamber itself, and the less 

 pressure must lie round the periphery. Such a distribution of 

 pressures would be set up if each flagellum caused a How of 

 water directed away from its own cell and towards the centre of 

 the chamber ; and this would be true whether the flagellum 

 beats synchronously with its fellows or not. 



The comparative study of the canal systems of sponges - 

 acquires a greater interest in proportion as the hope of correlat- 

 ing modifications with increase of efficiency seems to be realised. 

 In a few main issues this hope may be said to have been realised. 

 The points, so to speak, of a good canal system are (1) high 

 oscular velocity, which ensures rapid removal of waste products 

 to a wholesome distance ; (2) a slow current without eddies in 

 the flagellated chambers, to allow of the choanocytes picking up 

 food particles (see below), and moreover to present injury to the 

 delicate collars of those cells; (3) a small area of choanocytes, 

 and consequent small expenditure of energy in current production. 



It is then at once clear at what a disadvantage the Ascons 

 are placed as compared with other sponges having canal systems 

 of the second or third types. Their chamber and oscular 

 currents can differ but slightly, the difference being obtained 

 merely by narrowing the lumen of the distal extremity of the 

 body to form the oscular rim. Further, the choanocytes are 



^ Vosmaer and Pekelliaring, J'c7-h. Ak. Amsterdam, 1898. 



~ See Bidder, P. Camb. Soc. vi. 1888, p. 183 : Sollas, ChaUenger Monngraph, 

 XXV. 1883, pp. xviii.-xxi. : and Vosmaer and Pekelliaring, he. c.it. 



