30 CCELENTERATA. 



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 medusae 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 radiating chymiferous tubes, 

 or canals, lead to the periphery of the disk, where they open 

 into the very delicate circular circumferential 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 chymiferous tubes into 

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

 more or less prominent according to maturity and the breeding 

 season. The eggs or spermatozoa, as the case may be, are de- 

 hisced from these into the water directly. 



6. The tentacles. Is their arrangement a radially symmetrical 

 one? How are the nematocysts arranged on them? Look for 

 adhesive organs on them. Of what use are such organs? 



Turn your specimen with the velum side toward you and 

 study the edge of the medusa with a low-power objective for 

 the sense organs. These are of two kinds: 



(a) The larger, round bodies at the bases of the tentacles 

 communicate with the circumferential canal (which may possibly 

 be seen along the edge of the bell). They are filled with a layer 

 of strongly pigmented endoderm cells and are probably light- 

 percipient organs. 



