38 



ZOOLOGY 



common form is somewhat flask 

 shaped. At its free end are 

 situated one or more slit-like 

 openings, the oscuta. The body 

 is compressed from side to side, 

 and its wall is pierced by 

 numerous minute inhalent pores, 

 which lead by a system of 

 braiiching tubes into the central 

 cavity. 



The substance of the sponge 

 is composed of three layers 

 the ectoderm, the endoderm, and 

 between them the mesoderm. 

 The ectoderm consists of flat- 

 tened cells covering the outside 

 of the sponge, and lining certain 

 pits or depressions which are 

 pushed into the substance of 

 the sponge, and are termed in- 

 tercanal spaces. The openings 

 from the exterior into the inter- 

 canal spaces are termed "pores" 

 ';5 (1, Fig. 29). Several pores 

 are usually grouped together, and 

 form the pore area. The inter- 

 canal spaces open on their inside 

 7 by numerous apertures, called 

 Fig. 29. -Part of a section through by SoHas " prosopyles," into the 



Grantia lahynnthica, vertical to flagellate chambers (4, Fig. 29). 

 the margin and to the two surfaces ° , 



of the wall of the cup. After ihese iiagellate chambers are 



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. 



the most characteristic feature 

 of the Sponges. They are lined 

 by collared flagellate cells similar 

 to those of the Choanoflagel- 

 lata. Their flagella keep up 

 a constant current of water, 

 which passes in at the pores 

 through the intercanal spaces and flagellate chambers. 



