96 THE EVOLUTION OF CONTINUITY 



gonophore's must have its fluid contents kept in circulation, 

 and it is probable that the rhythmical contractions of the 

 bell not only result in locomotion, but also keep the canal 

 circulation going. 



Like the sea-anemone (though only part of an Indi- 

 vidual), the gonophore reveals its zooidal derivation by 

 producing several sets of reproductive organs, and these 

 arise in the expected situations. For they appear in the 

 walls of the radiating canals — that is, in the walls enclosing 

 what represent ancestral body-cavities or diverticula ; or 

 else they appear in the walls of the manubrium, itself 

 equivalent to a compressed zooidal series. 



At the margin of the bell tentacles are situated, and a 

 thin veil of tissue projects inwards, the " velum," narrowing 

 the bell-orifice after the manner of a ring-diaphragm. At 

 the base of the velum there are an upper and lower nerve 

 ring. 



The primitive medusiform gonophore was no doubt 

 permanently attached to the parent colony, through whom 

 it was fed, and for this reason was not called on to form a 

 mouth, and this form of gonophore is typical of several 

 present-day Hydrozoa. But a higher form evolved in which 

 a mouth was formed, and in which the bell by rhythmical 

 contractions acted as an organ of locomotion bearing the 

 gonophore away to lead a free existence. It is possible 

 that occlusion of the canal in the stem attaching the gono- 

 phore to the colony may have been a cause as well as an 

 accompaniment of strong bell-contractions and separation, 

 as insufficient nourishment and oxygen, and the accumulation 

 of waste products in the organism's canals, could cause the 

 bell-contractions to begin. These could in turn, by raising 

 pressure, cause a mouth to open, when rhythmical " respira- 

 tion " and locomotion would naturally result. 



We do not believe that the medusiform gonophore is a 

 specially modified zooid ; it is of the sporosac that this is 

 true. We hold that the medusiform gonophore is equivalent 

 to a sporosac-gonoblastidium which, as it attempted to 

 develop, has been terminally compressed to form a mega- 

 zooid. So far, it would seem, no attempt has been made to 

 explain the real significance of the medusiform gonophore, 



