28 "MORPHOLOGY OF INVERTEBRATE TYPES 



over and continue to the edge of the bell where they open into a 

 circular canal. The stomach with its system of canals represents 

 the ccelenteron of the medusa. It is usually known under the 

 name of gastro-vascular system. The broad, contorted bands 

 accompanying the radial canals are the gonads. Around the edge 

 of the medusa are thirty-two tentacles; of these four long tenta- 

 cles are at the points where the radial canals open into the circu- 

 lar canals and four halfway between them, eight medium long 

 ones halfway between the preceding and sixteen short tentacles 

 each halfway between the long and medium ones. The tentacles 

 are hollow and their cavity communicates with the circular canal. 

 Each tentacle is swollen at the base and is therefore subdivided 

 into a bulb and shaft. Between each pair of tentacles are three 

 small protuberances or rudimentary tentacles of which there are 

 therefore 96 all together. There are 128 simple marginal sense 

 organs or statocysts, alternating with the tentacles and protuber- 

 ances. The entrance to the subumbrella is somewhat constricted 

 by a flat, circular diaphragm or "velum. The velum is the organ 

 of locomotion and when it contracts a pressure is produced on 

 the water in the bell-cavity, which forces the medusa to move in 

 the direction of its apex. 



It is evident that the medusa is built on the principle of radial 

 symmetry. The axis of the bell, passing through the apex or 

 centre of the exumbrella and the mouth is the longitudinal axis. 

 The bell may be divided into symmetrical octants by four 

 planes intersecting in the longitudinal axis. Two of these planes 

 are called perradial and run through the radial canals; they 

 divide the stomach diagonally. The other two planes, called 

 interradial run halfway between the radial canals; they divide 

 each side of the stomach in two. A plane dividing two opposite 

 octants into two halves is called adradial. There are therefore 

 four adradial planes, but neither of them divides the medusa into 

 two symmetric halves. In accordance with the above termi- 

 nology the tentacles receive the names of the planes to which 

 they belong. There are four perradial, four interradial and 



