146 INVERTEBRATE ZOOLOGY 



color of the animal. Two species of the genus Hydra are 

 common in this country, H. viridis and H. fusca, the first of 

 which is green, the second, brown. 



Detach a polyp by means of a pipette, place it in a watch- 

 glass of water, and study it under a microscope. 



Exercise 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 endoderm. The cells of the 

 latter are much longer and more irregular than those of the 

 ectoderm ; their inner surfaces are amoeboid and also often 

 flagellate. Imbedded in the endoderm 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 algse living symbioti- 

 cally with the polyp. Between these two cell layers is a 

 thin non-cellular one called the supporting layer. The tentacle 



