Freshicater Medusa Liiniiocuida taiigauicce. 305 



have changed their liabitat from salt to fresh water, just as 

 the IIyih"oi<l Cordi/lopliora has done. Even if tlie cliange of 

 hiihitat oriniiially took place in the Niger, and not in Lake 

 T.innaiiyika, we shoulil still have to find the means of 

 conveyance across the African continent. Mr. Boulenger, in 

 liis Presidential Address to the Zoological Section of the 

 British Association at its meeting last year in South Africa, 

 appears to me to have clearly shown the road (' Nature/ 

 Aug. 190-5, p. 417). Paijeontological evidence points to the 

 fac^t that a sea extended over the greater part of Africa above 

 the Equator during a part of the Eocene period. " On this 

 retreating northwards after the Lutetian period, Medusc-B 

 became land-locked and gradually adapted themselves to 

 fresh water.'' With a sea stretching across the Soudan one 

 can account for the presence of Limnocnida in the Niger and 

 in the Great Lakes. It removes the need to speculate about 

 the Medusae ascending the Niger from the Atlantic and 

 migrating across Africa. 



Oiir knowledge of the life-history of Limnocnida is due to 

 Ml-. Moore's observations in Lake Tanganyika. In his 

 *' Tanganyika Problem" lie states tiiat the sexually mature 

 individuals swarm during September and October. The ova 

 and spermatozoa are evacuated, and he found " numbers of 

 small planulae and small Medus;^ which were growing 

 rapidly ; but these showed no tendency to form buds during 

 the autumn, and had, without doubt, been formed from the 

 fertilizi'd ova of the sexual forms." Mr. Moore states clearly 

 that tiie Medusa reproduces only by direct development, and 

 has no intervening liydroid stage. 



It appears t) me that the weakest part in the chain of 

 evidence for direct development is the connexion between the 

 })laiuila; and the young Medusae. Granting that the planuUe 

 belonged to the M' dusa and not to some other animal, there 

 is no mention made of the very important stages between the 

 planula stage and the young fully-formed Medusa. Tliese 

 are just the stages of wliich we require a full account, as they 

 are likely to give a clue to the relationship of this peculiar 

 ^ledusa to other members of the group. 



Tiie presence of young Medusie late in the year, when the 

 sexuiil atlults are breeding, is a common occurrence among 

 those Mc<lu.-ie of our seas which belong to species known to 

 have an intervening hydroid stage, i'liese young Medusa3 

 are late arrivals, either from a hydroid or from a Medusa 

 which reproduces asexually by g-Mumation, and they usually 

 die off without reproducing. 



