U4 



PORIFERA SPONGES. 



species. It is traversed by canals, through which currents of 



water bear food inwards and waste outwards. Numerous 



minute fores on the surface open into afferent canals, leading 



into a cavity or cavities lined by endoderm cells, many or all 

 of which are flagellate. To the activity 

 of the flagella the all-important water 

 currents are due. The endodermic or 

 gastric 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. Where there is a dis- 

 tinct central cavity there is usually but 

 one large exhalant aperture (osculum), 

 but in other cases there are many 

 exhalant apertures. 



The ectoderm is the least developed 

 layer ; it covers the body, and is perhaps 

 continued into the afferent canals. The 

 endoderm lines most of the internal 

 cavities, and is typically flagellate. The 

 intervening mesoglcea contains a skeleton 

 of lime, flint, or spongin ; amoeboid cells 

 or phagocytes, important in digestion and 

 excretion ; reproductive cells, and other 

 elements. 

 Budding is very common, and in a feiv cases buds are set 



adrift. Both hermaphrodite and unisexual forms occur. The 



sexually -produced embryo is 



almost always developed within 



the mesogloza, and leaves the 



sponge as a ciliated larva. With 



the. exception of one family, all 



are marine. 



Description of a simple 



sponge. — A very simple sponge, 



such as Ascetta, is a hollow vase, 



moored at one end to rock or 



seaweed, with a large exhalant 



aperture at the opposite pole, and with numerous minute 



inhalant pores penetrating the walls. These walls con- 



FiG. 48. — Simple 

 sponge {Ascetta 



primordia lis). — 

 After Haeckel. 



Note the vase-like form, 

 the apical osculum, the 

 inhalant pores in the 

 walls. 



Fig. 48A. — A sponge colony. 



