SERTULARIA, GONIONEMTJS 49 



numbers 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 po- 

 sition 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 exumbrella (aboral), while the concave portion is termed 

 the subumbrella (oral) . 



2. The velum is the perforated diaphragm that partly 

 closes in the subumbrella. All medusae possessing this 

 structure are classed as Craspedota. Do you understand its 

 use? 



3. In the center of the subumbrella is seen the large pen- 

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

 mouth. What is the shape of the mouth when open? How 

 does it compare in this respect with the mouth of Hydra? 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 perradii. Do you understand the use of these 

 canals? 



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