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. 
