82 



INVERTEBRATE ZOOLOGY 



cells. The cells covering the surface of the body and those 

 lining the internal cavities have an entirely different history 

 from the ectoderm and entoderm cells of other many-celled 

 animals. For this reason many zoologists maintain that the 

 sponges represent a line of development wholly independent 

 of all the higher animals and consequently set off the sponges 

 from the Metazoa, applying to this phylum the rank of a separate 

 subkingdom, the Parazoa. Certain chambers inside the sponge 

 are lined with flagellated cells which, because of their location 

 and function in capturing and digesting food, have frequently 

 been considered as entodermal cells. In Fig. 49, the location 





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B 



Fig. 49. — Morphological types of sponges. A, Ascon type; B, Sycon type; C, 

 Leucon type. {From Korschelt and Heidcr). 



of the flagellated cells is indicated by the hairlike parallel lines. 

 In the course of embryological development these flagellated 

 cells have been shown to arise from those cells of the embryo 

 which in normal development give rise to the ectoderm. In 

 fact, in the protozoan genus Proterospongia (Fig. 50 B), which 

 is thought by many to be like the ancestor bridging the gap 

 between the flagellate Protozoa and the sponges, the collared 

 flagellate cells are found on the outside of the body. 



Morphological Types. — Three types of sponge structure are 

 usually recognized (Fig. 49). In the simplest sponges, of the 

 Ascon type (A), the body is a thin-walled sac having a single 

 large opening, the osculum, at one extremity and numerous 

 minute incurrent pores through the body wall communicating 

 with a central cavity. This central cavity, which is frequently 

 called a stomach, is lined with flagellate cells, but since digestion 



