376 The Animal Kingdom 



Characteristics of the Phylum. — Except for a very brief, motile, 

 larval stage, the sponges are entirely sessile (attached) animals. Inas- 

 much as they cannot actively pursue or search for their food, they must 

 rely upon sifting food from water passing through the body. The vari- 

 ous cells composing the sponge are ideally adapted to this method of 

 obtaining food, for there are no special organs or tissues for the per- 

 formance of any functions. 



The outer epithelial cells are quite flattened in most forms and form 

 a pavementlike layer over the body surface. Among some sponges, some 

 of the epithelial cells are perforated with central pores which lead into 

 the central body cavity ; in others, the pores are openings to canals which 

 lead from the outside to the inner cavity and are lined with epithelial 



cells. 



The collar cells or choanocytes form the inner layer of the body 

 wall. Typical choanocytes are found only in the animals of this phylum. 

 Each cell is rounded and has a contractile collar at its free end (Fig. 

 123,D). From this collar projects a long flagellum which is structur- 

 ally like the flagellum of the protozoan class Flagellata. 



The mesoglea is a gelatinous-like matrix located between the cho- 

 anocytes and the epithelial cells. This material gives support to the 

 body and contains several different types of cells which correspond to 

 mesenchymal cells in other animals. Basically these cells are undif- 

 ferentiated amoebocytes which are able to wander through the matrix 

 to various sites. They are able to give rise to various types of cells, 

 but once they have differentiated into a specific cell type, they lose their 



plasticity. 



Depending upon the material to which they give rise, the amoeboid 

 cells are given different names. Some give rise to the skeleton of the 

 animal and are known as scleroblasts. Collencytes have slender branch- 

 ing processes which often unite to form a network ; myocytes are con- 

 tractile cells which are arranged as a sphincter at the main openings; 

 and the archeocytes remain relatively undifferentiated. Some give rise 

 to the reproductive cells, some are important in regeneration, and 

 some apparently are important in the nutrition of the animal. Although 

 there are many other types of amoebocytes which play roles of varying 

 importance, there are never any cells for the conduction of nerve 

 impulses. 



The skeleton of the sponge which arises from the scleroblasts is 

 composed of units known as spicules (Fig. 124,^ ) . These are variously 

 shaped in the different species. The three classes of sponges are dif- 



