PHYLUM PORIFERA 



Classes. 

 Hexactinellida. 



2. Demospongise 



Orders. 



' 1. Myxospongida. 



2. Ceratosa. 



3. Monaxonida. 



4. Tetractinellida. 



5. Lithistida. 



6. Octactinellida. 



\ 7. Heteractinellida. 



3. Calcarea (Calcispongiae). 



Sponges vary greatly in form, size, and complexity of 

 structure. A simple type is similar to a vase or hollow sac, 

 fixed by the lower end, and with an opening or osculum 

 at the upper extremity. The wall of such a sponge is thin, 

 and perforated by a- large number of pores through which 

 water flows into the central or g astral cavity and passes 

 out by the osculum ; by this means the sponge is provided 

 with food and oxygen and gets rid of waste matters. The 

 wall of the sponge consists of two layers — an outer or 

 dermal and an inner or gastral ; the dermal (fig. 7, 2) is 

 formed of a surface-layer of flattened cells, with a gelatinous 

 layer beneath containing various cells, some of which 

 secrete the elements of the skeleton. The gastral layer 

 (fig. 7, 3) consists of a single layer of cells, each cell being 

 provided with a collar-like projection, in the centre of 



