PENNARIA TIARELLA McCRADY 21 



manubrium of the male. The medusa has the shape of an elon- 

 gated bell or thimble. Its outer surface is called the exumbrella, 

 the inner surface the subumbrella. The opening of the bell is 

 partly closed by a circular "velum. From the apex of the subum- 

 brella the manubrium hangs down into the subumbrella cavity 

 like the tongue of a bell. The mouth is at the free end of the 

 manubrium. At the free edge of the bell are four equidistant 

 knobs rudimentary tentacles. Both exumbrella and subum- 

 brella are covered with ectodermal cells. The manubrium is 

 hollow and its cavity leads directly into a central cavity which 

 is known under various names such as the central gaslrovascular 

 cavity or the stomach and which represents the central part of 

 the coelenteron. Radiating from the central cavity are four 

 radial canals which run in the wall of the bell toward its edge 

 where they open into a circular canal. Four gonads develop in 

 the ectodermal layer of the manubrium at the time of maturity. 

 The microscopic structure of the medusa of Pennaria tiarella is 

 in its essential features the same as in Tima and will be con- 

 sidered in connection with that species. 



Instructions 



1. Place a complete colony of polyps,?, e., a hydrosome, in 

 a shallow dish with water and examine it under the dissecting 

 microscope. Make a half page drawing showing the hydrorhyza 

 or root, the hydrocaulus or stem with its branches and their 

 ramuli, and the hydranths or polyps. Label the founder polyp at 

 the top of the stem and the end polyps of the branches in the 

 sequence of their age as ai, bi, Cj . . . . Label the polyps of 

 two branches in the sequence of their age as a 2 , a 3 , a 4 . . ., b 2 , 

 b 3 , b 4 . . . . 



2. Examine a small branch under low power (50 diameters). 

 Make a half page drawing showing two hydranths of which at 

 least one should have medusa buds. Label perisarc or membrane 

 covering the stem, coenosarc or the wall of the stem; on the 



