36 MORPHOLOGY OF [NVERTEBRATE TYPES 



transverse fission a column or strobila of about a dozen cphyra. 

 The ephyra? begin to separate in March or April, appear soon 

 as small medusae, grow rapidly and become mature in summer. 

 Medusa. The medusa is built on the principle of radial 

 symmetry. It has the shape of a disc or better of a round con- 



FIG. 7. Aiirelia hibiata Ch. & Eys, from Mayer's Medusas of the World. 

 One mouth-arm has been cut off to show the structure of the medusa. 



vex concave lens. The oral surface or subumbrella is very slightly 

 concave. The aboral surface or ex umbrella has almost the shape 

 of a hemisphere when the medusa is fully contracted. The longi- 

 tudinal axis running from the centre of the subumbrella to the 

 centre of the exumbrella is shorter than the radius of the disc. 



