CHAPTER IV 

 PHYLUM PORIFERA 



The word sponge usually calls to mind the sponges of commerce 

 which are only one type of the skeletal remains from animals 

 belonging to the phylum Porifera. A mass of parenchymatous 

 tissue penetrated by numerous pores covers this framework in 

 the living animal. The living commercial sponges, though 

 slimy to the touch, have a fairly solid fleshy body which is 

 described by one author in the following manner: " In appearance 

 and consistency and the manner in which it cuts with a knife, 

 a living sheep's-wool sponge is not unlike a piece of beef liver, 

 perforated with holes and canals." Sponges vary so much in 

 shape that little can be said regarding their form. Without 

 exception, they are attached to some object and have lost powers 

 of free locomotion. Along with the lack of locomotor powers 

 there is the correlated lack of nervous tissue. In fact, there are 

 no specialized organs of any kind in the sponges. 



Distribution. — Sponges live under the most varied conditions 

 of aquatic existence from polar regions to the tropics and in 

 both salt and fresh water, but they seem to be more influenced by 

 conditions of the immediate environment than by mere geograph- 

 ical location. Some species are characteristic of the shore hnes, 

 even flourishing between tide marks, while others live only in 

 the greatest depths of the ocean. Only a single family (Spon- 

 gillidae of the class Demospongia) has become adapted to life in 

 fresh- water, but in the species of this family distribution is so 

 much facilitated by the reproductive buds or gemmules that 

 most of the genera are practically cosmopolitan. 



Fossil sponges appear in all strata of the Earth's crust back to 

 the Cambrian but even the oldest fossils are fairly closely similar 

 to forms of today, so they throw little light upon the problem 

 of the origin of sponges. 



Cell Layers. — The cell layers of the sponges offer but httle 

 opportunity for comparison with the remainder of the Metazoa. 

 The main bulk of the body consists of a solid mass of mesoderm 



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