FORM OF A SPONGE 303 



drawing water through the perforated membrane on which 

 they stand ; from its position the walls of the cloaca cannot bo 

 so perforated, so it bears no collar-cells. It is true that sponge- 

 fiagella can accelerate a current down which they lie, but for 

 this their position is least useful when they stand on a wall 

 parallel to the current, and most useful on a wall at right 

 angles to it. To this latter position they become more and 

 more limited in the progressive development of the canal 

 system, in which flagellate tubes become progressively shortened 

 until only the perforated hemispherical end of the chamber is 

 left. 



The remarkable achievement of the perfected hydraulic 

 organ in sponges is that from this waving of hairs xoFboo 

 of an inch in thickness at a mean speed of 7 feet an hour, 

 there is produced an oscular jet with an axial velocity 

 of over half a foot a second (280 times the speed of the flagel- 

 lum), which in Leucandra throws to the distance of 9 inches 

 five gallons a day or a ton in six weeks (Appendix, Note 3). 



It is, of course, clear that when we combine many slow 

 streams into one narrow channel the velocity is increased. 

 I computed in several sponges the aggregate cross-section of the 

 stream through the body in various parts of its course. In the 

 Leucandra of Text-figs. 1 and 7, 10 cm. long, there were 

 about 2j million flagellate chambers, with a total transverse 

 area of 52| sq. cm., or 1,700 times the area of the osculum, 

 which is 0-031 sq. cm. ; so that the mean velocity at the osculum 

 is 1,700 times as high as in the chambers. 



At 8-1 cm. a second, a quarter of a cubic centimetre (0-26 c.c.) 

 will issue from the osculum each second, and to replace it 

 a quarter of a cubic centimetre must have passed through the 

 21 million chambers, that is -geVo of a cubic millimetre through 

 each, or 116,000 cubic ^ ; which through a chamber 54 m diameter 

 (transverse area 2,300 sq. /^) implies a rate of flow of 50 m 

 a second, or tyVo ot that at the osculum, as above. 



The flagellate chamber (Text-fig. 7) is a blind, thimble- 

 shaped tube, the water entering through perforations in the 

 walls. The total area of the w^all-surface is eight times the cross- 



