Freshwater Crustacea of the Indian Archipelago. 247 



The above list suggests the following remarks and con- 

 clusions : — 



To the relative paucity of Entomostraca allusion has 

 already been made. Even though they are undoubtedly 

 more numerous than this list would lead us to suppose, they 

 are nevertheless of very much less frequent occurrence than 

 in European waters. Branchipus and Apus too are absent; 

 for, even if Entomostraca liave not been described hitherto 

 from the region with which we are dealing, still the two 

 genera in question, had they been present, must by reason of 

 their size have attracted the attention of previous investigators 

 also. But neither did von Martens meet with them in the 

 Indian Archipelago, nor did Semper observe them in the 

 Philippines. 



An important lesson is taught us by the Isopods. 

 In the first place, Asellidas are entirely wanting, as also 

 Spha^romidffi, certain species of which are likewise known 

 from fresh water. The xohole of the species observed hy me are 

 exclusively stationary or jjermanent parasites, and belong either 

 to marine specici^^ to marine genera, or, at least, to marine 

 families. 



Of Cymothoinse our list includes Ichthyoxenus Jelling- 

 hausii, Herklots, Rocinela typus, Milne-Edwards, and Tachma 

 lacustris, sp. n. 



Ichthyoxenus Jellinghausii was previously found in Java, 

 and I met with it myself in rivers in Sumatra upon species 

 of Puntius. A second species, Ichthyoxenus montanus, 

 Schiodte and Meinert, is known from rivers in the Himalayas 

 as parasitic upon Puntius sophore. 



A single specimen of Rocinela typus, Milne-Edwards, was 

 met with by von Martens in Borneo, in the Kapuas River 

 near Siutang, upon Notoptei-us hypselonofus, a freshwater 

 fish. I obtained numerous specimens from Cyprinoids in the 

 Lake of Singkarah, at an altitude of 362 metres above the 

 sea. Before this Rocinela typus was collected in the Bay of 

 Bengal. 



It is certainly by no means hazardous to suppose that this- 

 species, which belongs to an entirely marine genus, is also 

 capable of sojourning upon such marine fishes as visit the 

 mouths of rivers. It can be proved that Rocinela typus 

 temporarily leaves the fish in order to live upon the bottom, 

 for the sake of reproduction and perhaps also of undergoing 

 ecdysis. Jn seeking out a new fish our parasite may easily 

 attach itself to one of the numerous river-fishes which at the 

 same time also visit the brackish estuary, and thus gradually 

 for generations adapting itself to fresh water become a fresh- 



