30 CCELENTERATA. 



throughout the summer in the border of eel-grass in the Eel 

 Pond at Woods Hole, where it may be obtained with a dip-net. 

 It is more satisfactory to study than the medusa of Obelia, as 

 it is much larger and its movements and organs are more easily 

 observed. In plan of structure the two are quite similar. 



Put a living specimen in a jar containing sea-water, or in 

 a finger-bowl, with a black tile beneath, and notice : 



1. Its method of locomotion. To the contraction of what 

 part of the bell is movement due? How large is the jet of water 

 that is delivered from the bell? Why is the jet made narrow? 

 Does the jet necessarily leave at the center or may it be thrown 

 from one side? Should it be thrown from one side, what would 

 be the result? 



2. Its position in the water when quiet. Why is this position 

 more desirable than the opposite? With a needle-point prove 

 that various parts of the body are sensitive. 



With either fresh or preserved material notice: 



1. Its flattened dome-shape. The convex face is called the 

 ex-umbrella (aboral), while the concave portion is termed the 

 sub-umbrella (oral). 



2. The velum is the perforated diaphragm that partly closes 

 in the sub-umbrella. All medusse possessing this structure are 

 classed as Craspedota. Do you understand its use? 



3. In the center of the sub-umbrella is seen the large pen- 

 dent manubrium, at the extremity of which is a wide-lipped 

 mouth. If the medusa is alive, feed it with small bits of clam 

 meat. 



4. From the capacious sac at the base of the cavity of the 

 manubrium, the stomach, the four radial canals, lead to the 

 periphery of the disk, where they open into the very delicate 

 circular canal. The four radii marked out by these canals 

 are called the per-radii. Do you understand the use of 

 these canals? 



5. The gonads hang from beneath the radial canals into the 

 sub-umbrellar space. They are lobulated in structure, and 

 more or less prominent according to maturity and the breeding 



