THE POSITION OF SPONGES, ETC. 439 



tinuous layer lining the whole interior with the exception of 

 a narrow rim round the opening of the osculum, where they 

 are absent. In other sponges the collar cells form a discon- 

 tinuous layer, being confined to certain parts of the canal 

 system, the flagellated chambers. But however distributed 

 they are never absent, at least not so in any sponge that 

 has been properly examined. Not only are the collar cells 

 a constant peculiarity of sponge structure, but they also 

 occur nowhere else in the animal kingdom except in the 

 case of the Choanoflagellata, where we meet with them as 

 separate individuals leading a free existence, sometimes iso- 

 lated, sometimes united to form colonies, but in no case 

 losing their distinct individuality. 



The agreement in form and appearance between the 

 columnar collar cells with their long single flagellum sur- 

 rounded by the delicate protoplasmic, retractile collar, and 

 the individuals of the Choanoflagellata, is very striking. 

 The variations in the size, form, and appearance of the 

 collar cells in different sponges, are not greater than the 

 variations found amongst the species of Choanoflagellata, 

 if indeed they are as great. The physiological similarity 

 seems to be on a par with the structural resemblance. It 

 is certain that the collar cells take up food particles, and it 

 is probable that this is effected in the same manner as in 

 the case of the Choanoflagellata, namely, through the 

 particles being swept by the flagellum into or upon the 

 collar. Polejaeff (1883, p. 15) has tried to show that the 

 collar cells are not fitted for capturing food particles on the 

 ground that the lashing movements of the flagellum would 

 tend to drive the particles away ; but any one who has 

 observed the action of the flagellum in the free Flagellates 

 knows that it is exerted in a manner which would not have 

 this effect. An Euglena, for instance, when swimming has 

 its flagellum in front, and the body is dragged after it, as 

 it were ; if now an animal so moving were held fast by its 

 tail and fixed in one spot, as are the Choanoflagellata and 

 the collar cells of sponges, the consequence of the move- 

 ments of the flagellum would be to direct a current of 

 water towards the base of the flao-ellum, that is into the 



