IX CANAL SYSTEMS FOOD 237 



aphodal and diplodal chambers are as efficient as many of the 

 eurypylous type. 



It is manifest that the current is the bearer of the supply 

 of food ; but it requires more care to discover (1) what is the 

 nature of the food; (2) by which of the cells bathed by the 

 current the food is captured and by which digested. The 

 answer to the latter question has long been sought by experi- 

 menters/ who supplied the living sponge with finely powdered 

 coloured matters, such as carmine, indigo, charcoal, suspended in 

 water. The results received conflicting interpretations until it 

 became recognised that it was essential to take into account the 

 length of time during which the sponge had been fed before' its 

 tissues were subjected to microscopic examination. Vosmaer and 

 Pekelharing obtained the following facts : Spongilla lacustris 

 and Sycon ciliatum, when killed after feeding for from half an 

 hour to two hours with milk or carmine, contain these substances 

 in abundance in the bodies of the choanocytes and to a slight 

 degree in the deeper cells of the dermal tissue ; after feeding for 

 twenty-four hours the proportions are reversed, and if a period of 

 existence in water uncharged with carmine intervenes between 

 the long feed and death then the chambers are completely free 

 from carmine. These are perhaps the most conclusive experi- 

 ments yet described, aiid they show that the choanocytes ingest 

 solid particles and that th-e amoeboid cells of the dermal layer 

 receive the ingested matter from them. In all probability it is 

 fair to argue from these facts that solid particles of matter 

 suitable to form food for the sponge are similarly dealt with 

 by it and undergo digestion in the dermal cells. 



Choanocytes are the feeding organs far excellence; but the 

 pinacocytes perform a small share of the function of ingestion, 

 and in the higher sponges where the dermal tissue has acquired 

 a great bulk the share is perhaps increased. 



In the above experiments is implied the tacit assumption 

 that sponges take their food in the form of finely divided solids. 

 Haeckel^ states his opinion that they feed on solid particles 

 derived from decaying organisms, but that possibly decaying 

 substances in solution may eke out their diet. Loisel, in 1898,° 



1 Carter and Lieberkiihn in 1856, Haeckel in 1872, Metsohnikoff in 1879, and 

 many later workers. 

 2 Die Kalkschwiimme, 1872, i. p. 372. ^ /. Anat. Physiol. 1898, pp. 1, 6, 234. 



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