STRUCTURE OF SPONGES. 



125 



agree with the Ccelentera, and differ from higher (tri- 



ploblastic and ccelomate) Metazoa. 



The body varies greatly in shape, even within the same 



species. It is traversed by canals , through 



which currents of water bear food in- 

 wards 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 which the flag- 



ellum 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 outgrowing chambers, or it 



may be represented by branched spaces, 



from which efferent canals lead to the 



exterior. When there is a distinct central 



cavity there is usually but one large 



exhalant aperture (osculum), but in other 



cases there are many exhalant apertures. 

 A delicate outer layer covers the body, 



and is perhaps continued into the affer- 

 ent 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 ; amce.boid cells or phago- 

 cytes, important in digestion and 

 excretion ; reproductive cells, and 

 other elements. 



This median mass of cells is 

 traversed by the afferent canals 

 and by the diverticula of the 

 central cavity or the branches oj 

 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 lines it 



the endoderm, or the outer covering layer the ectoderm. In 



9 



FIG. 56. Simple 

 sponge (Ascetta 

 primordialis}. 

 After Haeckel. 



Note the vase-like form, 

 the apical osculum, the 

 inhalant pores in the 

 walls. 



FIG. 57. A sponge colony. 



