158 GENERAL ZOOLOGY 



skeleton, which may be composed of separate spicules of 

 lime or silica, or consist of a horny network. In their man- 

 ner of growth sponges are much like plants, living attached 

 to some object and varying their branches to suit the 

 surroundings. The natural history of a simple representa- 

 tive of the Porifera will give some idea of the general ac- 

 tivities of these peculiar metazoans. 



A SIMPLE SPONGE 



Self -maintenance. A sponge obtains food from the 

 water continually passing through its canals (Fig 65). 

 Little plants and animals are sucked into the pores from the 

 surrounding water, are drawn inside the collars of the choa- 

 nocytes or engulfed by other cells as they pass through the 

 canals. There is no digestive system, or digestive cavity, 

 no mouth or anus. A cell lining a canal takes food into 

 its cytoplasm much as an amoeba does; digestion takes 

 place only within the cells. The choanocytes and some 

 other cells lining the canals are, therefore, the only ones 

 able to feed, and they must pass on nourishment to the 

 rest of the sponge body. There is no circulatory system to 

 distribute food; transfer takes place through accidental 

 spaces between cells and by gradually soaking from one 

 cell into the next. Excretion occurs through exposed sur- 

 faces, and is greatly assisted by wandering amoeboid cells 

 which engulf waste particles and migrate outside the sponge's 

 body with them. 



Self -protection. Sponges are not very palatable morsels 

 for other animals, and hence have few enemies that seek to 

 devour them. They are usually protected by the sharp 

 spicules in all parts of their bodies. The spicules are ac- 

 cretions formed by cells which die after the work is completed 

 (Fig. 65, B, c). They not only protect the body, but also 

 help to support it. 



Sponges grow attached and hence cannot migrate to new 

 habitats if their surroundings become unfavorable. They 



