Relationships of the Porifercu 257 



being of great use to tlie whole colony, increased. Hence 

 arose a larger external layer of nutritive cells and a small 

 internal layer of now chiefly respiratory cells. 



This theory of Balfour's is criticized in Heider^s latest 

 work, and the arguments brought forward certainly seem to 

 us very powerful. " Balfour," says our author, '' was wrong- 

 in summarily dismissing the question whether we have not 

 perhaps in the amphiblastula-larva a cenogenetically modified 

 form." As such Heider considers it, especially as the amphi- 

 blastula is present only in the Calcarea, and not in all of these. 



Secondly, Heider thinks that we have yet no right to re- 

 gard the amoeboid cells as more proper to the reception of 

 nutriment than the flagellate cells. He points here to the 

 Salpingcecse and Codosigge, and maintains that our knowledge 

 of the mechanism of the motion of the flagellum is too slight to 

 enable us to form an opinion as to the powers of the collared 

 cells. In the third place, he objects that Balfour does not ex- 

 plain why the larva should have given up free movement. 



Heider now puts forward another hypothesis, based upon 

 his recent researches on Oscarella, wherein he assumes 

 " that the cavity of invagination is the gastral cavity, and 

 that the cells of the invaginated layer, thus in Sycon the 

 flagellate cells, were originally the nutrient elements." The 

 gastrula-like parent-form of the Sponges then gave up its free- 

 swimming mode of life " because it placed its mouth against 

 the surface of some solid body, in order in this manner to seek 

 food on the surfaces of stones swarming with minute organ- 

 isms of all sorts." The attachment took place originally 

 in the manner which Heider discovered in Oscarella, i. e. 

 only at single points, so that water could flow into the gastral 

 cavity all the time. There is certainly much to be said for 

 this hypothesis ; but if Heider objects that Balfour does not 

 explain why the ancestral form becomes attached and gives 

 up its free-swimming habit, we may, on the other hand, object 

 that Heider does not say why the blastula-like larva ever 

 turns into a gastiula. What was the principiiun movens in 

 this case? Everything appears to me to be still pure hypo- 

 thesis, to which one can only oppose other hypotheses. I 

 will willingly grant the possibility that the Metazoa may 

 have been derived from colonies of Protozoa. This is very pro- 

 bable, but not necessary j but so long as we do not yet know 

 which cells of the sponge and of the sponge-larva are nutri- 

 tive * and which subserve respiration, so long will it be of 



* Pol6jaeff considers it to be tolerably well showu that the collared 

 cells are very badly adapted to taking in food, and he supports this hypo- 

 thesis chiefly ou mechanical grounds. It must not be forgotten, however, 

 that as yet we know scarcely anything of micro-mechauics. 



