32 ANIMAL LIFE 



tion. They can perform all the other life processes ; they 

 move the colony by lashing the water with their flagella ; 

 they take in food and assimilate it ; they can feel. All the 

 cells of the great colony, too, are intimately connected by 

 means of protoplasmic threads. The protoplasm of one 

 cell can mingle with that of another cell; food can go 

 from cell to cell. The question whether the Volvox colony 

 is a group of distinct organisms or is a single organism 

 made up of cells among which there is a simple but obvi- 

 ous difference in structure and function ; in other words, 

 whether Volvox is a colony of one-celled animals, of Pro- 

 tozoa, or is a mnlticellular animal, one of the Metazoa (for 

 so all the many-celled animals are called), is a difficult one 

 to decide. Most zoologists class the Volvocinae with the 

 Protozoa that is, they incline to consider Gonium, Pan- 

 dorina, Volvox, and the other Volvocinae as groups or col- 

 onies of one-celled animals. 



20. Sponges. If the Volvocince be considered to belong 

 to the Protozoa, the sponges are the simplest of all the 

 many-celled animals. Sponges are not free-swimming ani- 

 mals, except for a short time in their young stage, but are 

 fixed, like plants. They live attached to some solid sub- 

 stance on the sea bottom. They resemble plants, too, in 

 the way in which the body is modified during growth by 

 the environment. If the rock to which the young sponge 

 is attached is rough and uneven, the body of the sponge 

 will grow so as to fit the unevenness ; if the rock surface is 

 smooth, the body of the sponge will be more regular. Thus 

 a sponge may be said to have no fixed shape of body ; indi- 

 viduals of the same species of sponge differ much in form. 

 The typical form of the sponges is that of a short cylinder 

 or vase attached by one end and with the upper free end 

 open (Fig. 17). Many individuals of one kind usually live 

 together in a close group or colony, and they may be so 

 attached to each other as to appear like a branching plant. 

 This branching may be very diffuse, and the branches 



