154 



PHYLUM PORIFERA — SPONGES 



The body varies greatly in shape, even within the same 

 species. It is traversed by canals, through which currents 

 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 

 which the flagellum rises (" choano- 

 cytes "), To the activity of the fiagella 

 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 ex- 

 terior. When there is a distinct central 

 cavity there is usually but one large 

 exhalant aperture {psculum), but in 

 other cases there are many exhalant 

 apertures. 



A delicate outer layer covers the 

 apical oscuium, the inhai- bodv , and IS perhaps inturucd iuto the 



ant pores in the walls. rr , 7 t^ 7 .1 



afferent canals. Beneath the covering 

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

 mesogloea) which may be very varied in its composition . Thus 

 there are scleroblasts making the skeleton of lime, flint, or 

 spongin ; amoeboid 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 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 lines 

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

 In fact, the sponges are very different from other Metazoa, 



Fig. 72. — Simple sponge 

 {Ascetta primordialis). 

 — After Haeckel. 



Note the vase-like form, the 



Fig. 73. — A sponge colony. 



