and Reproductive Phenomena of the Amoeban Rhizopods. 459 



coming putrescent. The decadence of every Rhizopod dates 

 from the commencement of this process; and there seems reason 

 to believe that the putrescence of the medium in which they 

 live is the only condition against which nature has furnished 

 them with no safeguard — in short, that the entire extermination 

 of the brood takes place whenever such putrescence has become 

 fairly established. 



Some of the aspects under which the Amcehce resist partial, 

 if not complete, desiccation have already been noticed. The 

 following additional example, however, has a twofold interest ; 

 for, on the one hand, it illustrates the nature of the relation 

 between the animal and the state of the medium by which it is 

 surrounded, and, on the other, serves to explain what has long- 

 been regarded by myself, and probably by many other observers, 

 as a very anomalous occasional condition of the DiatomaceaB. 



Some Diatoms would seem to be endowed with an increased 

 motile power, to assume a deeper colour from the inordinate 

 thickening of the endochrome-layer, and to generate an undue 

 quantity of oily matter, as soon as the water sustaining them 

 begins to be putrescent. Their healthy growth and multiplica- 

 tion are inseparably associated with an abundant supply of 

 oxygen and light. In common with the rest of the lower Algse, 

 they are frequently to be found in the same localities as the 

 Amoeban Rhizopods; and, like the latter, they are provided 

 with special means for resisting the extinction of their kind to 

 which they are consequently liable. But at a certain point, 

 although by no means so readily as the Rhizopods, they suc- 

 cumb in like manner to the action of putrescence ; and it seems 

 probable that the increased motile power and accumulation of 

 endochrome referred to are evidences of an expiring effort to 

 tide-over this condition, should it prove of a transitory nature. 



Some species certainly resist decomposition more successfully 

 than others; but there is no ground for supposing that this 

 increase of power signifies anything more than habituation 

 to conditions to which other species, or the same species when 

 inhabiting localities uninfluenced by such conditions, are not 

 amenable. The genus Pinnularia affords a notable example of 

 the kind, both from the circumstance of its occurring in the same 

 habitats as the Amoeban Rhizopods, and from its being frequently 

 of a size to render it admirably fitted for observation. 



The appearances about to be described have been seen by me, 

 not only in this country, but in the tropics and sub-Arctic 

 regions, always, however, in streamlets and pools such as those 

 referred to, and in connexion with Naviculoid Diatoms, as for 

 example, Pinnularia, Epithemia, Navicula, Stauroneis, and more 

 recently Nitzschia ; so that they are by no means uncommon. I 



30* 



