Coelenterata. 321 



of these parts separate from the colony. But certain buds 

 differ from the above. Usually near the base of the 

 colony are to be found longer buds which develop differ- 

 ently. They become long and club-shaped. The soft 

 cenosarc, blastostyle, develops circular side buds, called 

 medusa buds, which finally become detached and swim 

 away, passing out of a hole which is formed at the end of 

 the gonotheca, as the perisarc is here called. 



Medusae. The medusa bud when developed becomes 

 an umbrella-shaped body, and is called a medusa, or often 

 a hydromedusa. It is, in fact, a small kind of jellyfish. 

 The outside of the umbrella is called the exumbrella and 

 the inside the subumbrella. Hanging from the center of 

 the subumbrella surface is a short handle, the manubrium. 

 It is hollow, and the opening at its end is the mouth. The 

 cavity extends up through the handle, and continues as four 

 radiating tubes to the margin of the umbrella. These 

 four radiating tubes, or canals, are connected by a circular 

 canal, running around near the margin. Food is taken in 

 at the mouth, digested, and circulated through this system 

 of canals. From the margin of the umbrella a short fold 

 or shelf extends inward horizontally, and is called the veil 

 (velum). Around the margin are numerous tentacles, and 

 on the margin certain spots supposed to be organs of a 

 sense of direction. Some medusae have around the mar- 

 gin of the umbrella a series of black spots, which are rudi- 

 mentary eyes. The umbrella has the power of expanding 

 and contracting, and by this means the medusa swims. 

 Sometimes it turns inside out. 



How a Medusa Multiplies. Suspended from the sub- 

 umbrella surface, close to the four radial canals, are four 

 spherical bodies. These are the gonads. In one indi- 

 vidual the gonads produce eggs (ova), and in another 



