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.) 



Order 3. Cestida. 



Ribbon-shaped. Two tentacles with sheaths, and 

 numerous other tentacles. (Cestus.) 



Order 4. Beroida. 



Laterally compressed. Without tentacles. 

 (Berce.) 



Mayer: Mudusse 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 common fresh-water ccelenterate, is frequently 

 found 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. 



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? 



