NATURE STUDY IN SCHOOLS. 



inn 



The tubes not only answer to the respiratory system inasmuch as water 

 containing air is constantly passing through them, but this same water also 

 carries food with it and thus supplies the sponge with nutriment. 



The second set of tubes, as stated, are larger than the incurrent openings, 

 in fact they are considerably enlarged in places, forming chambers. ( See 

 fig. 83). From the sides of these chambers protrude peculiar cilia, each of 

 which is made up of three parts. ( 1 ) A base which is more or less cylin- 

 drical and hollow. See fig. 88, b. Protiuding from this is ( 2 ) a smaller 



Fig. 85. 



Tubular form of sponge. Upper figure single tut e. lower, cluster, size reduced. 



cylinder, having an open top. This is known as the collar cell ( B. ) From 

 this is thrust out ( 3 ) a slender whip-like organ which is the true cilia ( B ) 

 These collar cells and their accompanying bases and cilia are very numerous, 

 being packed closely together all over the surface of the chambers. See fig. 

 88 A, where is given an ideal section of the cilia chamber of a sponge. These 



