FUNCTION AND STRUCTURE 69 
tact with the food particle, and the body substance simply 
incloses the food (Fig. 3). Food is taken in by the sur- 
face. The whole outer surface of the body is the food- 
taking organ. In the simplest many-celled animals, the 
sponges, there is no special food-tuking and digestive organ. 
Each of the cells of the body takes in and assimilates food 
for itself, The sponge is like a great group of Amabu 
holding fast to each other, but each looking out for its own 
necessities. Among the m 
polyps, however, there i 
is a definite organ of 
digestion—that is, food 
is only taken and di- 
gested by certain parts 
of the body. The sim- 
ple polyp’s body (Fig. 
31) is a cylinder or vase 
closed at one end and 
open at the other end, 
and attached by the 
closed end to a rock. 
The opening is usually 
of less diameter than 
the diameter of the 
Fie. 31.— Obdelia sp.,a simple polyp; vertical sec- 
body, and it is sur- tion, highly magnified. 2m, mouth opening; 
al. s., alimentary sac.— After Parker and 
rounded by a number ft 
of tentacles, whose 
function it is to seize the food and convey it to the mouth 
opening. There are, of course, no teeth, no tongue, none 
of the various parts which are in or are part of the mouth 
of the higher animals. The polyp’s mouth is simply a 
hole or opening into the inside of the body. This body 
cavity, or simplest of all stomachs, is simply the cylindrical 
or vase-shaped hollow space inclosed by the body wall. 
This space extends also into the tentacles. There is no 
other opening, no posterior or anal opening. We can not 
6 
