39 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 Volvoz 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 Volvoz is a colony of one-celled animals, of Pro- 
tozoa, or is a multicellular animal, one of the Metazoa (for 
so all the many-celled animals are called), is a difficult one 
to decide. Most zodlogists class the Volvocine with the 
Protozoa—that is, they incline to consider Goniwm, Pan- 
dorina, Volvox, and the other Volvocine as groups or col- 
onies of one-celled animals. 
20. Sponges.—If the Volvocine be considered to belong 
to the Protozoa, the sponges are the simplest of all the 
many-celled animals. Sponges are not free-swimming ani- 
rials, 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 surfacc 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 (lig. 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 
