ZOOLOGY AND BOTANY, MIGEOSCOPY, ETC. 411 



two groups persist throughout the remainder of the ye;ir, though there 

 is a replacement of species and genera. Periodical incursions of Dia- 

 tornaceae take place throughout the summer, hut these are of short 

 duration until the late autumn, when the diatoms again appear in 

 numbers, and the plankton consists of a combination of Diatomacese 

 and Peridinieae. These conditions persist until the eud of December, 

 the period constituting the time of the greatest wealth of species. The 

 author is able to confirm the statement that the Peridiniese are luminous, 

 having observed this in Geratium fusus and Peridinium lenticulare be- 

 neath the Microscope. 



New Fresh-water Organisms.* — Max Voigt describes some new 

 forms found in the vicinity of the Biological Station at Plon. Among 

 them are a new Protozoon — Zachariasia velifera g. et sp. n., which 

 occurs only on Closterium ehreribergi ; and two new species of Gastro- 

 tricha, Chsetonotus serraticaudus and Ch. nodicaudus. 



Mollusca. 



Aquatic Mollusca of Switzerland, f — Dr. 0. S. Imhof points out 

 that certain lacustrine genera, especially Anodonta, JJnio, and Limnsea, 

 are exceedingly variable, the varieties being, as he believes, directly 

 related to the variations in the external conditions due to storms, 

 amount of sediment in the water, and so on. He gives very full lists 

 of species and varieties for the different lakes, with tables showing the 

 distribution in the Swiss area. 



7. Gastropoda. 



Pleurotomaria beyrichi. J — E. L. Bouvier and H. Fischer have 

 been able, on a well-preserved specimen of this interesting mollusc, 

 to make some observations on the soft parts. They find that the gills 

 are symmetrically arranged in the branchial chamber, but the left is 

 larger than the right. Even this left gill, however, is small, and the 

 two gills scarcely occupy the anterior half of the pallial chamber. In 

 themselves they would in all probability be insufficient for the respira- 

 tory needs of the organism, were they not supplemented by an accessory 

 breathing-organ. This consists in a richly vascular area of the pallial 

 chamber, which is supplied by a large vessel, and is apparently truly 

 homologous with the lung of Helix. This vascular area and the gills 

 appear to take about equal parts in the respiratory process, and the 

 authors believe that this is the primitive condition. In the aquatic 

 Prosobranchs the gills predominate, and the vascular area becomes the 

 mucous gland ; in the terrestrial forms, on the other hand, the vascular 

 area becomes increasingly important, and, while retaining to some extent 

 its power of secreting mucus, forms the essential breathing-organ, while 

 the gills degenerate. 



The same authors continue their study of this interesting type,*; 

 and describe the alimentary tract and nervous system. They discuss 

 the hind part of the intestine, which shows a remarkable branch, 



* Zool. Anzeig., xxiv. (1901) pp. 191-5. 



t Biol. Centralbl., xxi. (1901) pp. 43-62. 



t Comptes Kendus, exxxii. (1901) pp. 5S3-5. § Tom. cit., pp. 845-7. 



