24 CCELENTERATA. 



Order 1. Cydippida. 



Nearly circular. Two tentacles, each of which 

 may be retracted into a sheath. (Pleurobra- 

 chia, Mnemiopsis.) 



Order 2. Lobata. 



Compressed in the vertical plane. Two large 

 oral lobes. No tentacle-sheaths. (Deiopea.) 



OrderS. Cestida. 



Ribbon-shaped. Two tentacles with sheaths, and 

 numerous other tentacles. (Cestus.) 



Order 4. Beroida. 



Laterally compressed. Without tentacles. 

 (Berce.) 



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

 Nutting: The Hydroids of the Woods Hole Region. Bui. U. S. Fish. Com., 

 19, 1899. 



HYDROZOA. 



HYDRA. (Fresh-water Polyp.) 



Hydra, the 'only common fresh-water ccelenterate, is fre- 

 quently found in jars of water taken from quiet pools or sluggish 

 streams that contain lily-pads, decaying leaves, and other vege- 

 table matter. The animals may frequently be found by examin- 

 ing 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. 



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 sensi- 

 tive? How do you know? 



