GENERAL ZOOLOGY 



Fig. 9.6. Types of sponges, continued: Keratosa. A, living bath sponge, Spongia. B, 

 horny sponge, Hircima, in its natural coral reef habitat. (Photographs by John F. Storr.) 



of spicules, occur as fossils in Pre-Cambrian and later deposits; the evolutionary 

 history of sponges thus covers more than half a billion years, and no group 

 of animals has a longer fossil record. 



All sponges are marine, except for a very few fresh-water forms, and most 

 occur in shallow water. They are typically firmly attached to the substrate at 

 maturity, but there is a free-swimming, flagellated larva in the early develop- 

 mental stages. Because of their attachment and manner of growth, sponges 

 were first classified as plants. Later, because of their possession of flagellated 

 cells with collars, they were regarded as protozoan colonies. They were later 

 grouped with the Coelenterata, and it was not until the beginning of the 

 present century that they were finally given their current taxonomic position 

 as members of a unique and aberrant phylum. 



The most familiar example of a sponge is Spongia, the fibrous skeleton of 

 which is the natural bath sponge of commerce. A sponge of this type may be 

 regarded as a colony of individuals, although the boundaries of these in- 

 dividuals are indefinite. To understand the organization of sponges, we must 

 begin with forms much simpler than Spongia. 



General Structure. The Olynthus. The structure of sponges is best 

 explained by first describing the fundamental type from which all sponges 

 have probably arisen in the evolutionary history of the phylum. This type, 

 which is called the olynthus, was originally considered an adult sponge and 

 was given the generic name Olynthus. It is now known to be a late stage in 

 the development of certain species. An olynthus (Fig. 9.7) is a cylindrical 

 organism, attached at its basal end, with an opening, the osculum, at its free 



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