GRANTIA CILIATA FABRICIUS 13 



and endoderm. The mesenchyme is often called mesoderm, 

 but since it has not the same origin as the mesoderm of higher 

 animals, the term mesenchyme is preferable. 



Ectoderm. The outside covering of the walls of the 

 sponge and the lining of the incurrent canals is formed by a 

 single layer of ectodermal cells. These cells are polygonal in 

 shape, quite flat and belong to the type of pavemental epi- 

 thelium. In the incurrent canals larger cells are found scattered 

 irregularly among the common ectodermal cells. These larger 

 cells are perforated and are therefore called porocytes. The 

 pore of a porocyte is nothing but a prosopyle which has already 

 been mentioned. 



Endoderm. The lining of the atrium as well as of the 

 radial tubes is formed by a single layer of endodermal cells. 

 These belong to two different types. The cells lining the atrium 

 look very much like the ectodermal cells and belong to the same 

 type although they are of different origin. The cells lining the 

 radial tubes are called collar cells or choanocytes. They are long, 

 with a collar at their inner edge and a long flagellum protruding 

 far into the hollow space of the tube. Owing to these cells the 

 radial tubes have been also termed the "flagellated chambers." 



Mesenchyme. The mesenchyme is a gelatinous sub- 

 stance or mesoglcea containing cells and spicules. It fills out 

 the space between the incurrent canals and the radial tubes and 

 forms the cortex under the ectodermal layer on the surface of 

 the sponge. Some of the cells found in the mesoglcea look like 

 common connective cells with several processes and are called 

 collencytes. Others assume the shape of long spindles and, 

 being contractile, function as muscles. Such cells are found 

 surrounding the apopyles and the osculum where they form a 

 real sphincter. Some cells have the ability of amoeboid motion 

 and are called amcebocytes. Still other cells are said to have 

 nervous function. All these various types of cells are supposed 

 to be modified collencytes. Another type of cells found in the 

 jnesoglcea are the more or less round sderoblasts or cells produc- 



