CONTENTS. xiii 



CHAPTER XII. 



THE CRUSTACEA OF LAKE TANGANYIKA. 



PAGE 

 The crabs — Platythelphusa and Limnothelphusa — The prawns .... 279 



CHAPTER XIII. 



THE NEW TANGANYIKA POLYZOAN. 

 Aracknoidia ray lankesteri ........... 295 



CHAPTER XIV. 



THE TANGANYIKA MEDUSA. 

 General structure of this organism — Its apparent life history .... 298 



CHAPTER XV. 



THE SPONGES AND PROTOZOA OF LAKE TANGANYIKA. 



Spongilla moorei — -Affinities of Spongilla moorei — Spongilla tanganyiktz — Affinities 



of Spongilla tanganyika — Potamolepis welteneri — Livingstone's scum . . 309 



CHAPTER XVI. 



GENERAL CONSIDERATION OF THE NATURE OF THE HALOLIMNIC 



FAUNA. 



Recapitulation of certain matters discussed in earlier pages — The geo- 

 graphical isolation of the Halolimnic group — Its coexistence with the 

 normal African fresh-water fauna — The distinctive character of the Tan- 

 ganyika fishes — The general characters of the components of the normal 

 Tanganyika fauna — These are not structurally intermediate between those of 

 the general African fresh-water fauna and the Halolimnic group— The Halo- 

 limnic gastropods cannot be regarded as derivitives from any recognised 

 fresh-water types — Similar reasoning applies to the Tanganyika jelly-fish — 

 Impossibility of regarding the marine characters of the Halolimnic group as 

 having been brought about through convergence — Impossibility of regarding 

 the Halolimnic group as the relic of an old fresh-water stock — The presence 

 of the Halolimnic animals necessitates the supposition of a former extension 

 of the sea into Central Africa, and thereby throws light into the geological 

 history of the continent 325 



