26 CCELENTERATA. 



Hargitt, C. W.: The Anthozoa of the Woods Hole Region. Bui. Bur. 



Fish., xxxii, 1912, Doc. No. 788. 



: The Medusae of the Woods Hole Region. Bui. Bur. Fish., xxiv, 1904. 



Hargitt, C. W.: The Anthozoa of the Woods Hole Region. Bui. Bur. 



Mayer: Mudusae of the World. Carnegie Inst., Wash., 1910. 



Nutting: The Hydroids of the Woods Hole Region. Bui. TT. S Fish. 



Com., 19, 1899. 



HYDROZOA. 



HYDRA. (Ffcsh-watcr Polyp.) 



Hydra, the common fresh-water coelenterate, is frequently 

 fomid in quiet pools or sluggish streams that contain lily- 

 pads, decaying leaves, and other vegetable matter. The ani- 

 mals may frequently be found by examining the surfaces of 

 submerged leaves, but it is usually better to allow such material 

 to stand in glass jars for a day or two, as the animals then 

 tend to collect on the lighter sides of the vessels. They are 

 easily kept in balanced aquaria. 



Tixamine specimens in an aquarium and find what you can 

 about their mode of life. Do they form colonies? 



Place a specimen in a watch-glass of water and examine it 

 with a lens. 



1. What is its shape and color? Is it attached? If so, by 

 what part of the body? Notice the circlet of tentacles. How 

 many are there? Compare notes with others and see if all 

 have the same number. How are they placed? 



2. Does the Hydra move its body or tentacles? Is it sen- 

 sitive? 



3. Examine with a low power of the microscope and review 

 the above points. You may also be able to see the mouth 

 around which the tentacles are arranged. 



Make two drawings, one showing the animal expanded and 

 the other contracted. 



Place your specimen on a shde under a cover-glass that is 

 supported by the edge of another cover-glass and examine 

 with a high power. Be careful not to crush it. Notice: 



4. The outer layer, ectoderm. What is its color? Is it 

 continuous over the whole outer surface ? Does it vary in thick- 

 ness? Are the cells of which it is composed apparently all alike? 



