ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 873 



removing mechanically the bits of loosened or softened shell ; but 

 the author is strongly of opinion that an acid solvent is made to act 

 upon the shell dui"ing the process of boring. 



Action of Sea "Water on Molluscs.* — H. A. Coutance has ex- 

 perimented on the action of the salts contained in sea-water on 

 the mussel, periwinkle, and whelk, and Venus decussata. As the 

 result of these experiments, he finds that the saline elements of the 

 sea water act very differently on molluscs, and that every modification 

 in the composition of the water finally becomes fatal to the animals. 



Their greater or less resistance depends on their organization. 

 Bivalves resist better than spiral shells, and in these two groups the 

 results vary according to the different species. 



Salts of potash are less favourable to life than salts of magnesia, 

 and salts of magnesia are less favourable than salts of soda. Out- 

 side of the salts dissolved in sea water sulphate of soda seems to 

 possess a well-established preserving neutx'ulity. 



The death of bivalves is caused by a general weakening of the 

 muscles. As the muscles can no longer draw together or open the 

 valves, the animal is exposed to the unfavourable or poisonous action 

 of the element. 



MoUuscoida. 



Segmentation of Ascidians.| — E. van Beneden and C. Julin dis- 

 cuss the phenomena of segmentation in Ascidians and its relations to 

 the organisation of the larva. The investigation was begun at Lervig 

 in Norway, where the simple Ascidian Corella parallelogramma is verj 

 abundant and sexually mature in August and September. Since then 

 the study has been continued at Naples — especially on Clavelina 

 rissoana. 



As soon as the first karyokinetic figure is formed it is possible to 

 distinguish the sides, ends, and probably also the ventral and dorsal 

 surfaces of the gastrula, and, consequently of the larva. The first 

 I>lane of segmentation is along the median plane of the future 

 Ascidian. At the 8-stage the materials from which the right and left, 

 ventral and dorsal, anterior and posterior portions of the gastula are 

 to be found, are already localized in distinct blastomeres. The ecto- 

 derm is gradually separated from the endoderm, beginning at the 

 8- and ending at the 44-stage. Throughout the whole period of seg- 

 mentation the phenomena of cell-division proceed from beliind 

 forwards, in the sense that in all the cells the segmentation-grooves 

 first appear on the face of the cell which looks backwards ; whenever 

 a blastomere is about to divide, two systems of concentric circles are to 

 be seen on the sm-face of the cell ; these may be called the antipodal 

 systems. The inner — or polar — zone forms the most prominent, and, 

 as a rule, very homogeneous portion of the surface. Around it there 



* " Translatcfl and published in the last report of the U. S. Commissioner of 

 Fihh and Fisheries." Original source unknown. Cf. Amer. Natural., xviii. (1884) 

 pp. 945-8, and xvii. (188J{) pp. 1079-80. 



t Arch. Biol., v. (1884) pp. 111-2G (2 pis.). 

 Hei. 2.— Vol. IV. 3 M 



