Branch Porifera 19 



scopic animal and plant life is abundant. Sponges as we 

 ordinarily see them, either while living or after they have 

 been treated for commercial purposes, convey to us no 

 idea of the typical structure of the sponge. A bathing 

 sponge, for instance, instead of being a sponge individual, 

 is a colony of individuals formed by the budding and 

 branching of an original sponge individual. The typical 

 sponge has but one large crater-like opening at its top, and 

 several smaller openings on its sides. Water currents 

 enter the smaller openings, pass through the canals to an 

 interior cavity and escape through the large opening called 

 the osculum. Food contained in the water is absorbed by 

 the cells as it passes through the water-courses. The 

 outer covering of the body, the ectoderm, is a delicate 

 membrane composed of a single layer of flat cells. A 

 similar layer, the endoderm, lines the interior cavity and 

 the canals leading to it. Between these is the mesoderm, 

 some of the cells of which have developed horny fibres, or 

 siliceous or calcareous spicules, which serve the purpose of 

 a skeleton that is seen in commercial sponges, the living 

 cells having been rotted and washed away. 



