CLASSIFICATION 15 



provided for them, where they decay. The animal matter 

 in them is " beaten, squeezed, or washed out," and their 

 skeletons sent to market (Fig. 9). 



Geographic Distribution. — Fresh-water sponges are found 

 in streams and lakes in all the continents. Marine forms 

 are found in all seas and in all depths, from the shore between 

 tide-marks to the deepest abysses of the ocean. They are most 

 abundant in tropical waters. 



Geologic Distribution. — Silicious sponges were not uncommon 

 in the Cambrian Period, and are found in the formations from 



Fig. 8. — Bringing sponges from the vessels to sponge wharf at Key West. 

 (Report U. S. F. C, 1902.) 



that time on. They were abundant in the Jurassic and very 

 abundant in the Cretaceous of Europe; none have been found 

 in that of America. 



Important Biologic Facts. — Even in this low type there is a 

 differentiation of certain cells for certain purposes, as the 

 skeletal and reproductive cells. True sexual reproduction ap- 

 pears for the first time in the Porifera. Conjugation was noted 

 in the Paramoecium. 



Classification. — Sponges are of three kinds: (1) The calcar- 

 eous sponges, containing much lime. They are of little or 



