ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 37 



which at the same time remains relatively constant in other distinguish- 

 ing characters. The varieties may he classed under five types, three of 

 which are quite the same as those described for Europe under the names 

 pellucida, galeata, and gracilis, while two are peculiar to the American 

 lakes. The interest of the communication is increased by the fact that 

 the range of variation described is confined to the summer forms, which 

 are parthenogenetic. 



Nervous System of Crayfish.* — PL. Owsiannikow has investigated 

 the minute structure of the nerve-elements in Astacus fluviatilis, and the 

 general structure of the nervous system. He finds that the nerve-cells 

 possess a membrane, and contain primitive fibrils of varying calibre. 

 The smaller fibrils form a network about the nucleus, the larger, which 

 lie nearer the cell-periphery, unite to form a thick bundle which passes 

 out of the cell into the axis-cylinder. The nucleus possesses a firm 

 membrane, and contains one or two corpuscles. Almost all the nerve- 

 cells are unipolar, and give rise usually to one nerve only. In the 

 ganglia white and grey matter can be distinguished, and there are two 

 kinds of nerve-cells, the large and the small. The large cells give rise 

 to broad processes which become the fibres of the longitudinal com- 

 missure. These fibres may cross so that the right becomes the left and 

 vice versa. With the small nerve- cells are connected fibrils belonging 

 chiefly to the first pair of nerves in each ganglion. These fibrils are 

 also, owing to the branching of their processes, connected with the grey 

 substance of the right and left half of the chain, and in addition send 

 fibrils to the longitudinal commissure. The last-named fibrils, during 

 their passage through the ganglion, may give off branches which lie 

 chiefly in the grey substance of the same side. The nerve-fibrils which 

 enter the ganglia from the second pair of nerves mostly branch in the 

 grey substance without coming into connection with nerve-cells, but a 

 few unite with the small nerve-cells, and form fibrils which go to the 

 longitudinal commissure. In consequence of these relations, the first 

 pair of nerves in the crayfish must be homologised with the anterior 

 roots of the Vertebrate nerves, the second with the posterior roots. The 

 so-called third pair of nerves in the crayfish consists almost exclusively 

 of blood-vessels. Besides the connection indicated above between the 

 grey substance of the two sides of the ganglia, certain specially large 

 cells give off processes which connect the grey matter of the two sides. 



Annulata. 



Sipunculus nudus.f — S. Metalnikoff has made an exhaustive study 

 of the anatomy and histology of this Gejmyrean. Among the points 

 of interest made out, the following may be noticed. Pigment-cells occur 

 in varying numbers, not only in the skin but in all the organs of the 

 body except the muscles. The cells disappear during starvation, when 

 the body becomes perceptibly paler. They vary greatly in size and in 

 the number of their nuclei, and appear to possess considerable power of 

 locomotion. The author is disposed to regard them as parasitic amcobas 

 or rather plasmodia ; in not a few cases he found that they were entirely 

 absent. He finds that the diverticulum of the gut is large in young and 



* Mem. Acad. Impe'r. Sci. St. Petersbourg, x. (1900) pp. 1-29 (1 pi.). 

 t Zeittchr. wits. Zoo]., lxviii. (1900) pp. 261-322 (0 figs.). 



