ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 683 



and only tadpoles in an advanced stage of development (without 

 feet, but 4 or 5 cm. long) could be kept alive in a mixture of 2 

 litres of sea water and 2 litres of fresh water. Tadpoles of 10 or 20 

 days could not live in a solution of more than 10 to 12 gr. per litre. 



P. Bert subsequently directed attention to the results of his 

 investigations made some ten years ago, in which he demonstrated 

 that it is the chloride of sodium in sea water which causes the 

 death of fresh-water forms immersed in it. Bert was able to make 

 out the whole of the process, for he found that the action is due 

 to exosmosis in the region of the gills, the epithelium of which 

 becomes opaque, and the circulation of blood arrested; while this 

 obtains in animals whose body is covered with a mucous secretion, 

 we find that, where (as in frogs and toads) this secretion is absent, the 

 exosmotic action gives rise to desiccation, in consequence of which the 

 animal dies, after having lost from a quarter to a third of its weight. 

 Indeed, a toad may be killed by simply plunging one of its legs into 

 sea-water. On the other hand an eel, unless we remove the mucus 

 from some portion of its body, will live for a long time in sea-water. 



Animals gradually accustomed to the action of sea- water — such as 

 Daphnia — present a very interesting phenomenon ; when the fresh 

 water in which they live has been gradually brought to a degree of 

 salinity equal to one -third of that of the sea, they die quite rapidly ; 

 but, a few days later, fresh Daphnice appear, which have been de- 

 veloped from the ova of those that are dead. Here, then, we have 

 acclimatization, not of the individual, but of the species. This 

 phenomenon was first observed by Plateau, but Bert independently 

 convinced himself of its truth. 



Infusoria — such as Paramcecia — and diatoms of fresh water resist 

 perfectly a degree of salinity which kills fish and Crustacea. 



Dealing with the opposite case, the author found that marine 

 animals were killed by fresh water, owing to the absence of chloride 

 of sodium ; for the fresh water has a kind of exaggerated endosmotic 

 actioD, swelling out the gills of fishes, in which the circulation becomes 

 arrested, rendering opaque the transparent epithelial layers, and de- 

 stroying the contractility of the chromatophores of Cephalopods, and 

 the muscles of worms. 



Acclimatization experiments led here to similar results ; in other 

 words, the animals die rapidly when the salinity of the water is 

 reduced by one-third. Further experiments are being made on a 

 subject which is of great interest, not only as bearing on the physiology 

 of the epithelium, but on the general history of aquatic forms.* 



B. INVERTEBRATA. 



Radial and Bilateral Symmetry in Animals.j — H. W. Conn 

 points out that the relation of radial to bilateral symmetry among 

 animals is a question in regard to which there has been considerable 



* F. Plateau (torn, cit., pp. 467-9) also subsequently wrote claiming priority 

 over M. de Varigny's results by an article published in 1870 (Me'm. Couronne's 

 Acad. R. Belg., xsxvi. 1870), in wLich he described the effect of sea water ou 

 fresh-water Articulates and of fresli water on marine Crustacea. 



t Johns Hopkins University Circulars, ii. (1883) pp. 73-4. 



