Medusa from the River Niger. 33 



Biyozoa, whose mode of dispersal is not without analogy to 

 that of Hydroids. Their colonies or their statoblasts are 

 frequently met with upon the shells of mollusks, not only 

 upon stationary or slowly-moving bivalves, but upon Gastro- 

 pods like Paludina, which are very well able to crawl against 

 the stream *. 



A few months aw, when Dr. Meissner was examining; the 

 specimens of vEtheria in the Berlin Museum, he recognized 

 upon their shells statoblasts of Plumatella^, thus placing 

 beyond doubt the existence of these Bryozoa in the great 

 rivers of Africa (Nile, Senegal, and Niger), although they 

 were for a long time not to be found upon the Dark Con- 

 tinent, and were so difficult to discover that a naturalist so 

 able as Stuhlmann declares that he searched for them in vain 

 for two years and a half \. 



^Etheria, masses of which form in great rivers accumula- 

 tions comparable to oyster-beds, and which, like the latter, 

 resist the action of the water, furnishes an excellent basis for 



* I append a few instances of freshwater Biyozoa attached to Mol- 

 lusca : — Upon the shell of an undetermined species of Unio Leidy 

 discovered at Philadelphia the curious Urnatella gracilis, while upon 

 Unio drlphinus, Lea, Symphinota bialata, Lea, and Anodonta securiformis, 

 Say, Jullien found Norodonia from China and Cambodia. The same 

 naturalist records the occurrence of statoblasts of Plumatella upon speci- 

 mens of Unio collected by M. Chaper in the Madras Presidency. Pecti- 

 natella Carteri, Hyatt, and Hislopia lacustris, Carter, have been likewise 

 met with in India upon Paludina bencjalensis, Lamk. (J. Jullien, " Mono- 

 graphic des Bryozoaires d'eau douce," Bull. Soc. Zool. de France, x. 1885, 

 passim). Kraepelin, moreover, has given some extremely interesting 

 details as to the veritable symbiosis which appears to exist between 

 Plumatella prince ps, Kraep., var. spont/iosa, Kraep.,and Paludina fasciata, 

 O. F. Miiller. In the Bille and in the Elbe, in the neighbourhood of 

 Hamburg, thousands upon thousands (" Tausende und Abertausende ") of 

 Paludina are found covered with Plumatella, and resembling in conse- 

 quence so inany potatoes rolled about by the water. It is a remarkable 

 fact that in these localities scarcely a trace of the Bryozoon in question is 

 seen upon submerged stones. Moreover the Paludina, in burying them- 

 selves in the mud to hibernate, ensure the preservation of the statoblasts 

 with which they are covered, and which they also bring back to the 

 surface on the return of spring, at the period of development (K. Kraepe- 

 lin, " Die Deutschen Siisswasser-Bryozoen," Abhandl. a. d. Geb. der 

 Naturwissensch., herausg. v. d. Nat. Verein Hamburg, x. 1887, p. 121 of 

 the separate impression, pi. iv. figs. 113 and 114). 



A number of exotic fluviatile mollusks, and especially the large and 

 small species of Ampullaria, which are so widely distributed in hot 

 countries, should be examined from the point of view here considered ; I 

 am not aware that this has ever been done. 



t Meissner, " Beitrag zur Kenntniss der geographischen Verbreituno- 

 der Bryozoengattung Plumatella in Afrika," Zool. Anzeiger, xvi. p. 385. 



% F. Stuhlmann, " Fauna von Ost Afrika," Sitzgsber. Gesellsch. natur- 

 forsch. Freunde zu Berlin, 1890, p. 184. 



Ann. & Mag. N. Hist. Ser. 6. Vol. xiv. 3 



