GROWTH OF IDEAS 275 



ing way. First, the primitive unicellular protozoa 

 came together and formed crude social bodies, 

 clusters of cells that kept together, but had no 

 special division of labour. As all the members in 

 the cluster pressed to the surface, in order to 

 obtain their food, they came to form, not a solid 

 mass of cells, but a hollow vesicle with a wall 

 of cells. Then the first division of labour set in. 

 Certain cells, those that were situated at the 

 anterior pole, and so were better placed to receive 

 the floating food as the animal moved along, 

 became the eating-cells of the group ; they pro- 

 vided nourishment for the others, as the nutritious 

 sap circulated through all the cells in the cluster, 

 as we find in the case of the siphonophores. As 

 these feeding-cells multiplied rapidly at the fore 

 part of the animal, a depression was formed at that 

 pole of the body. In the end the ball or vesicle 

 was doubled in upon itself, until it came to have 

 the form of a cup with a double-layered wall. 

 Externally were the cells in the skin that effected 

 movement and feeling, and aSorded protection ; 

 inside, forming the internal wall, were the eating- 

 or stomach-cells. An opening remained at the 

 top — the opening of the cup or vase-like body. 

 The food entered by it : it was virtually the 

 *' mouth." Thus was formed a primitive multi- 

 cellular animal with division of labour. If we 

 imagine it attaching itself to the bottom by its 

 lower pole, we can see that it would easily become 

 a sponge of the simplest kind, a polyp, a coral, or, 

 detaching itself once more, a medusa. If we 



