122 



PH YL UM POIUFERA SPONGES. 



agree 



with tJie Cc&lentera, and differ from higher (tri- 

 filoblastic and avlomate) Metazoa. 



The body varies greatly in shape, 

 ITCH within the same species. It is 

 traversed by canals, through which cur- 

 rents of water bear food inwards and 

 waste outwards. Numerous minute 

 pores on the surface open into afferent 

 canals, leading into a cavity or cavities 

 lined by flagellate cells, many or all of 

 which have a goblet shape with a delicate 

 collar through ivhicJi the flagellum rises 

 (" choanocytes "). To the activity of the 

 flagella the all-important water currents 

 are due. The internal cavity may be a 

 simple tube, or it may have radially out- 

 groiving chambers, or it may be repre- 

 se?ited by branched spaces, from which 

 efferent canals lead to the exterior. 

 Where there is a distinct central 

 cavity there is usually but one large 

 exhalant aperture (osculuni), but in 

 other cases there are many exhalant 

 apertures. 



A delicate outer layer covers the body, and is perhaps con- 

 tinued into the afferent canals. Beneath the covering layer 

 there is in all but the simplest forms a mass of cells (the 

 mesoglcea] which may be very 

 varied in its composition. Thus 

 there are scleroblasts making the 

 skeleton of lime, flint, or spongin ; 

 amceboid cells or phagocytes, im- 

 portant in digestion and ex- 



cretion ; reproductive cells, and 

 other elements. 



This median mass of cells is 

 traversed by the afferent canals FIG. 55. A sponge colony. 

 and by the diverticula of the 



central cavity or the branches of the original central cavity, 

 lined by flagellate cells. It is difficult to call this cavity or 

 system of cavities the gut or enteron, or to call the layer which 



FIG. 54. Simple 

 sponge (Ascetta 

 p riinordialis}. 

 After Haeckel. 



Note the vase-like form, 

 the apical osculum, the 

 inhalant pores in the 

 walls. 



