142 PHYLUM CCELENTERATA 



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 ten- 

 tacles are the lithocysts, mmute 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 toward the manubrium. It is the prin- 

 cipal 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 a side view of the animal on the same scale, show- 

 ing 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. 



The hydroid generation of Gonionemus is a minute, solitary 

 polyp which lives attached to the bottom in shallow water ; it 

 will not be studied here. The polyp is only about one millimeter 

 in height and has four tentacles which can be extended two milli- 

 meters. It is thus very much smaller than the medusa, which has 

 a height of about nine millimeters and a diameter of about twenty. 

 The polyp forms new polyps by budding, but has never been 

 observed forming the medusae ; so it is not known how these 

 originate. 



