86 



INVERTEBRATE ZOOLOGY 



within the cytoplasm of a scleroblast but as the spicule grows it 

 ultimately extends the limits of the mother cell and breaks out 

 from its walls. In some other instances spicules are formed by 

 the combined action of a group of scleroblasts. 



In the classification of sponges, the nature and form of the 

 skeletal structures are of great importance. Spicules may be 

 formed of either calcareous or silicious material in a great variety 

 of patterns. A network of a substance called spongin (Fig. 

 51, 1) ,is another type of supporting structure formed by cells 

 called spongioblasts. This spongin, which is closely allied to 

 silk in its chemical composition, may appear either alone or 

 along with silicious spicules. 



Fig. 51. — Skeletal structures of sponges {after Schulzc and Maas). 1, spongin 

 fiber surrounded by spongioblasts;' 2-7, different types of spicules. {From 

 Hertwig's Manual of Zoology by Kingslcy, courtesy of Henry Holt and Co.). 



Economic Importance. — Aside from the market value of 

 sponges gathered for domestic use and utilization in the arts, 

 sponges have relatively little direct value. They have no im- 

 portant enemies, but many kinds of crustaceans, worms, and 

 other animals find shelter within their canals and some species 

 of sponges have been found only on the bodies of other animals, 

 especially crabs, where they apparently hold a symbiotic relation- 

 ship. The spicules seem to afford a great measure of protection, 

 for practically no animals feed upon sponges. Some of the boring 

 sponges do damage to the shells of molluscs into which they 

 burrow for protection. 



Outline of Classification 



Phylum Porifera.^^Many celled; body penetrated by pores communicating 

 with one or more internal chambers or canals, at least one of which is lined 



