A GUIDE TO THE SPONGES 



223 



(See Fig. i.). This sponge is typically vase-like in external form- 

 The circular opening at the top of the vase is known as the 

 osculum (osc.) in spite of the fact that it is excretory and has 

 neither structurally nor functionally the characteristics of a 

 mouth. 



The walls of the vase are perforated with numerous regularly 

 arranged openings or- pores (p.) which open directly into the 

 hollow interior of the sponge — called the paragastric or atrial 

 cavity (atr.). The walls are made up of three layers: ist, the 

 ectoderm, or outer layer; 2d, the endoderm, or inner layer; 

 3d, the mesoderm, or middle layer. 



The ectoderm (ect.) is a thin layer of cells, generally arranged 

 in mosaic form and known as "pavement cells." In the case of 

 this species, however, the walls of the cells have disappeared 

 and left the protoplasmic cell-contents continuous over the 

 entire surface of the animal. Such a layer is called a syncytium. 



The endoderm (end.) lines the paragastric cavity and is made 

 up of a layer of peculiar and characteristic cells called "collared 

 cells, " or choanocytes (c/^o.), found nowhere else among many-celled 

 animals. They are so called from a collar-like rim around the 

 outer edge of the cell out of which extends a long whip-like 



-mes 



FIQ. 8.— SECTION THROUGH SPONGE WALL 

 ect.. ectoderm ; iiies., mesoderm ; eitJ., endoderm ■,c/ii.>., choanocytes or "collared cells"; 

 Jla., flagellum. 



filament or flagellum (fla.). The continuous vibration of these 

 flagella produces a current by means of which the sea-water, 

 with its multitude of tiny animal and plant forms, is sucked in 

 through the pores. The organisms are then seized upon by the 



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