236 PORIFERA 



acting on a volume of water which they can only imperfectly 

 control, and it is no doubt due to the necessity of limiting the 

 volume of water which the choanocytes have to set in motion that 

 the members of the Ascon family are so restricted in size. The 

 oscular rim is only a special case of a device adopted by sponges 

 at the very outset of their career, and retained and perfected 

 when they have reached their greatest heights ; the volume of 

 water passing per second over every cross-section of the path of 

 the current is of course the same, therefore by narrowing the 

 cross-sectional area of the path at any point, the velocity of the 

 current is proportionally increased at that point. The lining of 

 the oscular rim is of pinacocytes ; they determine a smooth surface, 

 offering Httle frictional resistance to the current, while choanocytes 

 in the same position would have been a hindrance, not only by 

 setting up friction, but by causing irregularities in the motion. 



Canal systems of the second type show a double advance upon 

 that of the Ascons, namely, subdivision of the gastral cavity and 

 much greater length of the smooth walled exhalant passage. The 

 choanocytes have now a task more equal to their strength, and, 

 further, there is now a very great inequality between the total sec- 

 tional areas of the flagellated chambers and that of the oscular tube. 



Canal systems of the third type with tubular chambers are 

 an improvement on those of the second, in that the area of 

 choanocytes is increased by the pouching of the chamber-layer 

 without corresponding increase in the size of the sponge. How- 

 ever, the area of choanocytes represents expenditure of energy, 

 and the next problem to be solved is how to retain the improved 

 current and at the same time to cut down expense. The first 

 step is to change the form of the chamber from tubular to spherical. 

 Now the energy of all the choanocytes is concentrated on the 

 same small volume of water. The area of choanocytes is less, 

 but the end result is as good as before. At the wide mouth of 

 the spherical chamber there is nevertheless still a cause of loss of 

 energy in the form of eddies, and it' is as an obviation of these 

 that one must regard the aphodi and prosodi with which higher 

 members of the Demospongiae are provided. The correctness of 

 this view receives support, apart from mechanical principles, 

 from the fact that the mass of the body of any one of these 

 sponges is greater relatively to the total flagellated area than in 

 those sponges with eurypylous chambers ; that is to say, a few 



