ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 27 



rabbit. In both, the endodermal cells change their character sooner 

 than the mesodermal. The latter (according to hypothesis, sixteen 

 instead of two) scarcely yet show a bilaterally symmetrical arrange- 

 ment. Such an arrangement would first become manifest when each 

 moiety of eight cells further proliferated, and the whole mesoderm 

 assumed the shape of a horseshoe or crescent with its concavity 

 directed forwards. This stage corresponds to E. van Beneden's 

 ninth, or to Planorbis in Fig. 19 of Eabl's former paper. We thus 

 trace the initial resemblances among developing bilateral animals, 

 vertebrate or invertebrate. 



Five stages of the egg during segmentation, as seen from the 

 vegetal pole, are figured. All the figures are taken from P. mar- 

 ginatus, which, like P. carinatus, would seem to have smaller endo- 

 dermal cells (less crowded with yolk-granules) than P. corneus. 



Development of Paludinidae. — In a note to his previous paper, 

 Herr Eabl adverts briefly to the development of BitJiynia, the study 

 of which he began diiring the spring of 1880. In both genera the 

 primitive renal organs arise in the same way, each being produced 

 by the excavation of a large specialized mesodermal cell. Bobretzky 

 has mistaken for primitive kidneys parts of young prosobranchs 

 which are homologous with the lateral vacuolated regions of the 

 velum of Planorbis. 



The supra-oesophageal ganglion of Bithynia also originates just as 

 in Planorbis. 



The cells of the entoderm divide into cylinder-cells, albumen- 

 cells, and yolk-cells. 



Pedal Nervous System of Paludina vivipara.* — Dr. H. Simroth, 

 already known by his interesting researches into the mode of locomo- 

 tion of the MoUusca, here deals with an important factor of the 

 phenomena he has already described. The massive retractor j)edis of 

 the mollusc in question can be easily divided into two lateral halves. 

 The removal of these retractors can be effected without any re- 

 moval of the nerves — a certain proof that the nerves in question 

 expressly supply the foot itself. This foot is also rich in blood- 

 lacunae. Between the median one and the lateral there is a thick 

 branch of the pedal nerve, which diminishes in size as it passes back- 

 wards. Four commissures connect together the trunks which are 

 given off from the pedal ganglion, and of these the second is the 

 widest ; and it also is seen under the Microscope to give off nerve- 

 branches. The branches given off from the pedal nerves follow one 

 another with great regularity, and they can be separated into an in- 

 ternal and an external series. The first of the inner set has an inter- 

 esting course. After passing a little backwards and downwards, it 

 turns round and goes forward to supply one-half of the anterior edge 

 of the foot. It early divides into two branches, one of which has a 

 lateral course and is stronger than the median one, which runs almost 

 directly forwards ; these two branches are connected by anastomosing 

 ramules. 



* Zeitschr. wiss. Zool., xxxv. (1880) pp. 141-50 (1 fig.). 



