66 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



face of the body is covered by a single layer of flat- 

 tened cells forming the ectoderm. The canals are more 

 or less completely lined with a layer of cells, each 

 of which is provided with & flag e Hum, by means of which 

 water is propelled through the canals toward the central 

 cavity. Between these two layers is a mass of amoeboid 

 and other cells which compose the mesoderm, and in 

 which the skeleton, or framework, of the sponge is de- 

 veloped. The skeleton may be composed of flexible 

 fibers of spongin, as in the toilet sponge ; 

 or of spongin fibers together with spic- 

 ules of calcareous matter, as in Grantia ; 

 or of siliceous spicules alone, as in 

 the fresh-water sponge (Spongilla and 

 Myenia) and Venus's flower basket 

 (Euplectelld) ', or of spicules of carbo- 

 nate of lime and so on, while a few 

 have no skeleton at all. 



The flagellate cells are peculiar, in 

 that they have an upgrowth on their 

 free end, formed by a delicate expansion 

 of the cell substance, and having the 

 shape of a broad collar surrounding the 

 base of the flagellum, whence their 

 name of "collar cell" (Fig. 14). 



The water flowing in through the 

 canals bears with it the small particles 

 of organic material upon which the sponge feeds, the 

 particles being captured apparently by the collar cells 

 as well as by the amoeboid cells. The same currents 

 of water serve also for respiration. Reproduction is by 

 means of eggs and by budding, the latter process giving 

 rise to a group of connected sponges. The young larval 

 sponge which develops from the fertilized egg is pro- 

 vided with cilia, by means of which it can swim around 



FIG. 14. "Collar 

 cell " from Grantia, 

 showing flagellum, 

 f; " collar," c ; nu- 

 cleus, n ; contractile 

 vacuole, cv . Much 

 magnified. 



