TYPE C(ELENTERA. 



81 



which projects from the rim, and is known as the velum ; 

 from the presence of this structure these medusae have been 

 termed craspedote medusae. The cavity between this and the 

 bell is the subumbrellar cavity, into which projects a process 

 corresponding to the clapper of the bell and termed the 

 manubrium. At the free end of this is the mouth, which 

 opens into a canal traversing the mauubrium and communi- 

 cating at its base with the gastric cavity lying in the sub- 

 stance of the bell. From this four (in some cases more) 

 canals or pouches radiate out towards the rim and com^ 

 niunicate there with a circular canal which extends coni- 



FIG. 41. Liriope scutigera (after BROOKS). 



pletely round the margin of the bell, just where the velum 

 joins it. The canal of the manubrium, the gastric cavity, 

 and the radiating and circular canals together constitute the 

 ccelenterou. To the margin of the bell tentacles, varying in 

 number, are generally attached, on or between the bases of 

 which sense-organs are to be found. 



The medusae differ not only in form but also in habit from 

 the polyps, being free-swimming organisms, propelling them- 

 selves through the water by expelling the water from the 

 subumbrellar cavity through the velar opening by sudden 

 and rhythmical contractions of the bell. In accordance with 

 this free mode of life sense-organs are present, and a higher 



