138 



THE RISE OF ANIMAL LIFE 



osculum 



ompbiblosl'ula 



inversion 



Fig. 8-4. Anatomy and life history of a simple sponge. 



alga, Chlorella, living in the body cells. 

 Their most common habitat is some feet 

 below the tidal zone of the sea ( upper and 

 lower limits of the tide) (Fig. 8-1), al- 

 though some, like the glass sponge (Fig. 

 8-2), live as much as three and a half miles 

 below the surface of the ocean. Fresh-water 

 forms are also known, some of these attach- 

 ing themselves to twigs and rocks in the 

 streams (Fig. 8-3). 



Morphology of a simple sponge 



In order to have some knowledcre of the 

 morphology of the sponge, it is best to 

 discuss a simple sponge, Leucosolenia, as 

 an example. It lives beneath the low tide 

 level in the sea and consists of many slender 

 upright tubes which are joined at their 

 bases in a many-branched common tube 

 (Fig. 8-4). The upright portions are thin- 



