PORIFERA. 27 



In the living state a large number of small openings 

 termed pores (fig. 5, p) may be seen on the surface, these 

 in many forms are not permanent, disappearing and being 

 replaced by others, they are the openings of canals leading 

 to the interior, known as the incurrent canals (fig. 5, i). 

 These dilate into globular chambers (g), from which 

 canals pass to the central cavity of the sponge known as 

 the excurrent canals (e), their openings on the inner 

 surface being the ostia (o). The opening of the central 

 cavity to the exterior is termed the osculum (os\ in some 

 cases there is only one, which is then placed at the 

 summit of the sponge ; in others there are several. A 

 continual current of water flows through the canal-system, 

 passing in by the pores and out by the osculum. By 

 means of this the processes of respiration, nutrition and 

 excretion go on. 



The wall of the sponge is formed of three layers, 

 the outer one is the ectoderm and consists of a single 

 layer of flattened cells. The inner is the endoderm, it 

 lines the globular chambers and the excurrent canals and 

 is similar to the ectoderm, except in the chambers, where 

 each cell is provided with a collar and a flagellum, the 

 movements of the latter producing the current of water. 

 Between the endoderm and the ectoderm comes the 

 mesoderm which is much thicker than the other layers 

 and forms the greater part of the sponge; in it the 

 skeleton is produced. 



In nearly all sponges there is a skeleton, which serves 

 to support the soft parts and to prevent the canals from 

 closing. For systematic purposes its structure is found to 

 be of immense importance, more especially since the 

 external form of the sponge varies enormously even in the 

 same species. The skeleton may consist of fibres of a 



