176 INVERTEBRATE ZOOLOGY 



Directly beneath the four radial canals and projecting slightly 

 into the subumbrella space are the four reproductive organs, which 

 are also brown in color and present a corrugated appearance. 

 The sexes are separate, but the animals are not dimorphic. 



Observe the number and arrangement of .the tentacles, of which 

 an adult medusa possesses from sixty to eighty. Note the spiral 

 arrangement of the nettle cells on each tentacle, and also the 

 adhesive pad near its outer end. It is by means of the nettle 

 cells in these pads that the animal anchors itself to seaweeds 

 and other objects when at rest. Note the exact point above 

 the margin of the bell where the tentacles are inserted. In the 

 basal portion of each tentacle is a conspicuous pigmented body ; 

 this is a hollow bulb which is connected with the ring canal. 

 Between the tentacles are the lithocysts — minute projections 

 from the margin of the bell which are probably equilibrial in 

 function. 



Observe the velum — the membrane which extends around the 

 inner margin of the bell towards the manubrium. It is the 

 principal organ of locomotion and contains bands of ectodermal 

 muscle fibers by the contractions of which the motion of the 

 bell is produced which propels the animal through the water. 

 Similar bands of muscle fibers are also present in both the 

 subumbrella and the exumbrella. 



Exercise 1. Draw a semidiagrammatic view of the exumbrella 

 on a scale of from 5 to 10, showing the tentacles extended 

 and all the organs which have been observed. 



Exercise 2. Draw an oblique side view of the animal on the 

 same scale, showing the velum, the manubrium, and all the 

 other organs observed. 



Exercise 3. Draw a semidiagrammatic view of the subumbrella 

 on a scale of 5 to 10, showing the velum, the manubrium, 

 and the other organs observed. 



