HYDRA 43 



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



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

 : The Medusas of the Woods Hole Region. Bull. Bur. Fish., xxiv, 



1904. 

 Mayer: Medusas of the World. Carnegie Inst., Washington, 1910. 

 Nutting: The Hydroids of the Woods Hole Region. Bull. U. S. Fish. 



Com., 19, 1899. 



HYDROZOA 



HYDRA (Fresh-water Polyp) 



Hydra, the common fresh-water coelenterate, is fre- 

 quently found in quiet pools or sluggish streams that contain 

 lily pads, decaying leaves, and other vegetable matter. The 

 animals 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 ani- 

 mals then tend to collect on the lighter sides of the vessels. 

 They are easily kept in balanced aquaria. 



Examine 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 re- 

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

 mouth around which the tentacles are arranged. What is the 

 shape of the mouth when open? 



Make two drawings, one showing the animal expanded 

 and the other contracted. 



Place your specimen on a slide under a coverglass that is 

 supported by the edge of another coverglass and examine 

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



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



