PORIFERA 107 



fibrous, chitinous skeleton. The separate individuals of these 

 usually compound sponges are distinguished only by the oscula ; 

 the rest of the body is intimately united with its neighbors. 



Order 5, Myxospongida. — Skeleton absent (whence the name 

 from Greek myxa, mucus, + spongia, sponge). Unknown 

 in the fossil state. 



1. Since a protozoon consists of a single cell and a sponge of 

 many cells, why cannot the latter be considered a colony of the 

 former ? 



2. What is the principal advance of the Porifera upon the 

 Protozoa ? 



3. What is the characteristic skeletal element in sponges? 

 Give variations in its chemical composition and in its structure, 

 wuth examples of each. 



4. Amongst the different sponges, give stages of the gradual 

 replacement of spicules with spongin. 



5. Give the habitat of commercial or bath sponges. How 

 are they prepared for market ? Are they simple or compound ? 



6. What function did the various holes in the common bath 

 sponge have when the animal was living ? 



7. Why do the boring sponges penetrate shells? 



8. From the rocks of what period is the earliest record of 

 sponges ? 



