ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 763 



Influence of the External Medium on the Saline Constituents 

 of the Blood of Aquatic Animals.* — L. Fredericq points out that the 

 water of the North Sea contains a little more than 3 per cent, of 

 soluble salts, and has a marked salt and bitter taste ; the blood of the 

 Crustacea and Cephalopoda living in it has exactly the same taste, 

 which leads to the supposition that it has the same chemical consti- 

 tution ; and this view is supported by chemical analyses. The blood 

 of the crabs of the brackish water of Braeckman has a less salt taste, 

 and that of the crayfishes of the Belgian rivers still less so. It 

 would, then, seem to be certain that, in virtue of the laws of diffusion, 

 there is a more or less perfect exchange of salts between the blood 

 and the external medium, and the seat of this process is probably the 

 gills. But this variation in chemical composition, according to the 

 characters of the external medium, would appear to be confined to the 

 " lower animals " ; a similar diffusion might take place in fishes, but 

 in them we find that the saline constituents of the blood are very 

 different to those of the seas in which they live ; an explanation of 

 which may be found in the fact that the blood is in them much more 

 isolated from the surrounding medium than it is in the invertebrate 

 marine forms. 



B. INVERTEBRATA. 



Development of some Metazoa.t — In the third part of his studies 

 on Comparative Embryology, E. Metschnikoff states that he finds that 

 the identity between so called Archigastrulce does not exist as Haeckel 

 has supposed ; nor must an Archigastrula be always bilaminate, for 

 some very primitive forms contain mesodermal cells even during the 

 blastula stage. Indications of a radial structure of the archigastrulaj 

 have not been detected in the Echinodermata only ; doubly symme- 

 trical arrangements are not at first seen in the blastopore, and the 

 elongated form of that structure must not, when found, be regarded 

 as of palingenetic, but of adaptive origin. On the supposition that the 

 radial gastrula form is the primary one, the question arises whether the 

 gastrulas of Echinoderms, and of such forms as Linens and Polygordius, 

 are really homologous ; if they are so it would seem to be a necessary 

 consequence that the anus of the Echincpsedia is the homologue of 

 the pharyngeal orifice of the worm — a comparison with which the 

 author is apparently dissatisfied. 



It is a question whether the Gastraea theory affords the promised 

 key to the solution of morphological problems ; the author points out 

 that the formation of the endoderm, in many of the lowest Metazoa, 

 by the appearance of separate cells in the segmentation cavity, can by 

 no possibility be regarded as a compression of the process of invagina- 

 tion, though, on the other hand, the invagination seen in the higher 

 Ccelenterata may quite easily be regarded as a shortened process. If 

 we want to remain true to the Gastrasa theory, we must suppose not 



* Bull. E. Acad. Sci. Belg., iv. (1S82) pp. 209-12. 

 t Zeitschr. f. wiss. Zoo!, xxxvii. (1882) pp. 286-313. 



