90 ANIMAL INDIVIDUALITY [CH. 



them together so that each preserves a considerable 

 measure of independence, and is yet subordinated 

 to the good of the whole. This resulted in the 

 metazoan type of structure, where the individual 

 is built up out of a number of cells instead of one. 

 As an example of a simple metazoau, we had 

 better take a primitive sponge. Among sponges, 

 Clathrina blanca is one of the most primitive. A 

 graceful vase-like creature, pure white, with a long 

 stalk of attachment, and a mere fraction of an inch 

 in length, it obtains its food, like the majority of 

 sedentary aquatic animals, by producing a current. 

 A stream of water can easily be demonstrated passing 

 out of the circular mouth of the vase, and, with a 

 little more trouble, can be seen to enter by a number 

 of quite small holes scattered over the walls. The 

 current is produced by the cells lining the central 

 cavity (Fig. 6) : these stand side by side like sacks in 

 a granary, their free upper ends tapering very slightly, 

 and then truncated at the top. The flat top of each 

 is surrounded by the most remarkable transparent 

 cylinder, a mere film of protoplasm, yet beautifully 

 round, and capable of being drawn in at will, or 

 protruded till it equals the cell in length. This is 

 called the cottar, and in the centre of it there springs 

 from the cell a long vibrating lash or ftagettum, of 

 uniform thickness throughout, and also capable of 

 retraction within the body of the cell, ^sually 



