296 



ZOOLOGY OF JllE IN^ EKTEIUIA TA. 



layer usually associated with a limiting membrane, which surrounds 

 their body and gives it a definite shape. As a consequence of 

 the presence of this cortical layer, these forms, which take solid 

 food, have acquired one or more channels through which the 

 nutriment is''[ingested, and usually a definite area whence the 

 undigested remnants are extruded. 



The genus Froterospongia, 

 Fig. 73, one of the Flagellates 

 discovered by Saville Kent, in 

 which the individuals of the 

 colony are sunk in a jelly, lends 

 some support to the view that 

 Sponges may have originated 

 from colonies of Choano 

 flagellata. 



Most Flagellata live in fresh 

 water ; some are marine, and 

 some parasitic, living in the 

 alimentary canal or blood of 

 Vetebrates and Arthropods. 



Turning to the Metazoa we 

 abstract a short description, of 

 the sponges. 



Fig. 74. — Part of a section 

 through Grautia labyrijithica^ 

 vertical to the margin, and to 

 the two surfaces of the wall of 

 the cup. After Dendy. 



1. Inhalent pore. 



2. Exhalent canal. 



3. Inhalent canal. 



4. Cavity of flagellate chamber. 



5. Pore area. 



6. Gastral skeleton. 



7. Dermal skeleton. 



8. Tubar skeleton. 



9. Embryos. Fig. 74. — Grautia labyrinthica. 



The substance of the sjjonge is composed of three layers : the 



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