1907.] MEDUSA FROM AFRICAN LAKES. 645 



We are therefore compelled to the view that the asexual method 

 of reproduction is the most usual one in Lake Tanganyika through- 

 out the year, and that the sexual method may be confined to a 

 definite season ; the earliest date at which it has been observed 

 is in the month of Ma}^, and we have as yet no evidence for 

 its continuance beyond the month of July. 



It is much to be regretted that no light has hitherto been shed 

 upon the process of development from the egg. We have no 

 suflicient reason for believing that the organisms described by 

 Moore as the planulse or larvse of the Limnocaida really were 

 such, and the existence of any free hydroid stage is as doubtful as 

 ever. However, one fact of importance has been confirmed by all 

 observers, namely, that the medusa may suddenly appear on the 

 surface in covmtless numbers, in shoals many miles in length, and as- 

 suddenly disappear so that none are to be seen for a month or more. 



The excellent state of preservation of Dr. Ounnington's material 

 has enabled me to realise the natural appearance of a living 

 Limnocnida when swimming, more perfectly than when I received 

 Mr. Moir's first consignment of preserved material. The longer 

 and older tentacles are carried somewhat stifily above the exum- 

 bral surface of the medusa (PI. XXXVII.), while the smaller and 

 younger series of somewhat clubbed " velar" tentacles, as they are 

 sometimes called, curve round the umbral rim. Tentacles of 

 intermediate length occupy intermediate positions, and so the 

 living animal can erect over its back a very efiicient chevcmx de 

 frise armed with nematocysts for offence and defence. This 

 fashion of carrying the tentacles is like that adopted by Limno- 

 codiuni and Olindioides, in which latter form the tentacles adhere 

 to the exumbrella along a more considerable proportion of their 

 length than in Limnocnida, an adhesion Avhich affords greater 

 stability to the system. 



Another point on which Dr. Ounnington's collection throws 

 welcome light, is that of the succession and development of the 

 tentacles. While still attached to the parent, the young medusa- 

 buds develop the first two ordei'S of tentacles in the per- and inter- 

 radii (PI. XXXYII. fig. 7). The youngest free-swimming stages 

 in the collection, 2 millimetres in diameter, have the tentacles oJ 

 the fifth order just commencing to sprout (text-fig. 172). Between 

 these young stages and the oldest with tentacles of the eighth 

 order and 22 millims. in diameter, the intermediate stages are 

 faii'ly completely represented. 



The peculiar sense-organs first become conspicuous in young 

 medusas in which tentacles of the fifth order are appearing, but 

 they are not invariably present at this stage. In the youngest 

 animals in which they were detected, there were four (text-fig. 172) 

 in each quadrant, or 16 in all, although minor irregularities may 

 occur, as in the specimen shown in text- fig. 172, in which only 

 15 sense-organs were present, and thenceforth they increase in 

 number, until they are so numerous and crowded as to be almost 

 touching one another all round the circumference of the medusa 

 (text-fig. 174, S.O.). 



