22 THE PROTOZOA THE DAWN OF LIFE 



the flagellate, nutritive cells of the sponges are provided with a 

 collar similar to that of the Proterospongia and its allies, a 

 form of cell which does not exist in any other group of the 

 Metazoa. 



If we dip a collecting bottle into a pond or clear pool during 

 the summer months, we may sometimes see, on holding the bottle 

 up to the light, that its fluid contents is peopled by a number of 

 very small, brightish green spheres, which roll about through the 

 water in all directions, and which are specimens of the inter- 

 esting Volvox globator. Each hollow sphere represents a colony 

 composed of a single layer of individual cells, each contained in 

 its own cell-wall or capsule, united together by protoplasmic 

 bridges, the entire colony numbering from 1,500 to 22,000 cells. 

 Botanists and zoologists in the past fought long for the sole pos- 

 session of the Volvox, the former claiming it as belonging to the 

 algae, the latter equally firmly claiming it as a flagellate colonial 

 Infusorian ; for the Volvox has characteristics both vegetable and 

 animal. Its possession of flagella, contractile vacuoles, and eye- 

 spots attest to its animal properties, while the presence of chroma- 

 tophores, starch granules, and proteid granules (pyrenoids) demon- 

 strate its vegetable affinities. 



The life-history of this remarkable Protista has many inter- 

 esting points, which seem to foreshadow the more complex life 

 of the Metazoa or multicellular animals. In that half of the 

 Volvox hemisphere which is posterior in swimming, some five to 

 eight larger cells can be seen, which, as they grow, segment to form 

 new colonies. At first each young colony is a plate, but as its 

 cells multiply the plate bends up and finally forms a hollow sphere. 

 When at last the parent sphere ruptures for the liberation of the 

 young colonies, it sinks to the bottom of the pond and dies. This 

 is the first example of a progressive change leading to the intro- 

 duction of death as a constant phenomenon, as it occurs in the 

 higher animals. 



The ordinary Protozoon, as we have seen, is a single cell, and 

 forms no body. It divides, and in this way multiplies, but the 

 products of division go asunder. In most Protozoa division takes 

 place without any loss, there is no distinction between parent and 

 offspring, there is continual self-recuperation, and as there is no 

 body formed there is no death. Now in Volvox, the cell which 



