THE TANGANYIKA PROBLEM. 331 



The jelly-fish in Tanganyika, as Mr. Gi'inther and I 

 have shown, is possessed of a remarkable primitive type 

 of structure even among these forms. 



The sponges which are peculiar to Tanganyika, Spon- 

 gilla tanganyikac and S. moorei, might be regarded as 

 having originated from fresh-water types, but they could, 

 except for their fresh-water habitat, as Mr. Evans pointed 

 out, be just as well associated with the Chalinidae, a 

 marine family group, and like the prawns they are not 

 found outside the confines of the lake. Potamolepis, a 

 sponge common to Tanganyika and the Congo, is a highly 

 peculiar form, and the chief fact of importance, regarding 

 it in this connection, is that its spicules are identical 

 with those of the old fossil genus Renieria which occurs 

 extensively in the marine deposits of the secondary rocks. 

 It is precisely the same with the Protozoa of Tanganyika, 

 i.e., the remarkable Collpodium and Condylostoma peculiar 

 to that lake. 



In considering the meaning of the peculiarities which 

 characterise the constituents of the halolimnic group, 

 it is a fact of very great importance that the normal 

 fresh-water components of the fauna of Tanganyika are in 

 no way peculiar ; they do not in the least stand as structural 

 stepping-stones between the components of the general 

 African fresh-water fauna and the members of the halo- 

 limnic group. This point has been referred to again and 

 again while dealing with the anatomy of the different 

 organisms which constitute the halolimnic group. Hardly 

 any of these halolimnic types could, in fact, under any cir- 

 cumstances be regarded as modifications or specialisations 

 of any of the recognised African fresh-water types. The 

 halolimnic gastropods are quite incapable of being re 

 garded as derivatives from the recognised fresh-water 



