PHYLUM I. SPONGIDA (PORIFERA). 



Sponges differ from other animals in so many respects 

 that for a long time naturalists were uncertain as to whether 

 they were animals or plants, but this matter has long been 

 settled beyond dispute. There is, however, more ques- 

 tion as to the position of these forms 

 in the Animal Kingdom. They have 

 been regarded as colonial Protozoa, as 

 members of the Coelenterata, and as a 

 distinct group, or phylum, the position 

 given them here. 



The structure of a sponge can be 

 best understood by starting with the 

 simplest forms (fig. 10). One of these 

 is a vase-like structure with a central 

 or gastral cavity communicating with 

 the exterior by a terminal opening, the 

 osculum. Through the sides of the vase 

 are numerous small openings or pores 

 (whence the name Porifera) , and through 

 these water, carrying with it oxygen 

 and small food particles, is drawn, the 

 waste-water passing from the gastral 

 cavity by means of the osculum. Such 

 a sponge is the so-called Ascon type. 



If now the gastral cavity develop pouch-like folds in 



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FIG. 10. A simple 

 (Olynthus) sponge 

 (after Haeckel). One 

 side has been broken 

 away to show the 

 interior, e, spicules; 

 i, eggs ; o, osculum ; 

 p, pores ; u, gastral 

 cavity. 



