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TEXTBOOK OF ZOOLOGY 



derm comes to line the internal cavities and its function is circu- 

 lating the water. The middle layer is very poorly differentiated, 

 being hardly more than a matrix, and is hardly recognizable as the 

 mesoderm of the typical triploblastic animal. If the type can be 

 classified according to germ layers, it might be considered a modi- 

 fied diploblastic (two germ layers) form. Because of these pecu- 

 liarities, some authors have called sponges Mesozoa or Parazoa. 



Classification 



Class Calcispongiae. — Single, shallow-water, marine forms, char- 

 acterized by calcareous spicules. There are two orders. 



Fig. 49. — Glass sponge or Venus's flower basket, Euplectella sp., is probably the 

 most beautiful of the sponges. (Courtesy of General Biological Supply House.) 



Order Homocoela. — Simplest type, possessing a very thin body wall 

 with pores as perforations in individual cells. The internal cavity 

 is lined with choanocytes. Leucosolenia. 



