160 INVERTEBRATE ZOOLOGY 
color of the animal. Two species of the hydra are common in 
this country — the green hydra and the brown hydra, the latter 
of which has much longer tentacles than the former. 
Detach a polyp by means of a pipette, place it in a watch- 
glass of water, and study it under a microscope. 
Bxercise 1. Draw several outlines of the animal on a large scale, 
showing different shapes and positions it can assume. 
Notice the mouth in the midst of the tentacles. 
Place a polyp on a slide in water and cover it with a cover- 
glass, but support the corners of the latter with wax to avoid 
crushing the animal. Study its structure under the microscope. 
We see that the type of structure is radial and not bilaterally 
symmetrical; that the animal is tubular in shape, and that its 
internal cavity opens to the outside through the mouth; that 
the mouth is a small opening at the end of the conical, terminal 
portion of the body, called the hypostome, at the base of which 
are the tentacles. The internal cavity is called the gastro 
vascular space and is the common digestive and circulatory cavity 
of the animal. The end by which the animal is attached is 
called the foot. It is an adhesive disc containing gland-cells 
which produce a sticky secretion. 
Examine the finer structure under a high power of the 
microscope. The body-wall is made up of two layers of cells, 
the outer ectoderm and the inner entoderm. The cells of the 
latter are much longer and more irregular than those of the 
ectoderm; their inner surfaces are amceboid and also often 
flagellate. Imbedded in the entoderm cells are the chromato- 
phores, the bodies which give color to the animals. In the 
green hydra these are chlorophyll bodies; in the brown hydra 
they consist of a substance similar to chlorophyll. In both 
cases they are probably single-celled alge living symbioti- 
cally with the polyp. Between these two cell layers is a 
thin non-cellular one called the supporting layer. The tentacle 
