Sexton: On a collection of Gammarus from the Königsberg Museum. 91 
It was while examining some of the material first sent me from Königsberg 
that I noticed a difference in the appearance of specimens from different localities. This 
observation has since been amply corroborated by material from other sources. The 
difference in appearance seems to be connected with the degree of salinity of the water 
and to be confined to the epidermis and its outgrowths. Specimens from fresh water 
are more broadly built, with thicker chitin in the epidermis, and with the antennae. 
peraeopods, and pleon furnished with dense clusters of long fine hairs. The specimens 
from brackish water are slender, delicate in appearance, semi-transparent, with more 
spines developed, and only a few hairs. The finest brackish water specimens I have 
examined are in this collection, taken at Zoppot and Rauschen and in Norway. 
The species is a very widespread one. To the records of its distribution already 
given (P. Z. S. 1912. p. 658) there can now be added Norway, and the Crimea, as 
stated above. It has also been found by M. Edouard Chevreux, at Dunkerque, 
and at Nantes, forty miles up the Loire. 
The change in appearance in accordance with the environment is also shown 
but in a much less degree, in the two other species, e. g. the G. pulex from the 
Pregel, and the neighbourhood of Königsberg where the water is perhaps slightly brackish, 
have developed more spines, particularly on the pleon, than those from the freshwater 
Geserich-See. The G. locusta specimens are also affected by their surroundings. 
The distinguishing specific characters will be seen in the accompanying figures, 
the antennae, perhaps, forming the quiekest means of separating the species. In G. pulex 
the upper antenna has very few setae, and the accessory flagellum is short, 3-4 jointed ; 
the flagellum of the lower antennae in the male is provided with numeröus short straight 
setae, giving it a bottle-brush effect. In G. zaddachi the peduncles and flagella of both 
antennae carry dense clusters of long, graduated, outstanding setae : while in G. locusta 
the upper antenna is, as in G . pulex almost glabrous, but the accessory flagellum is 
long, 13-14 jointed, and the peduncle and flagellum of the lower antenna are furnished 
with tufts of long fine hairs. 
The specimens preserved in the Museum and examined are as follows, described 
according to the district in which they were taken. 
Ostsee. 
Memel. ”G. locusta. See b. Memel. 8 . 9. 75. no. 7294.” Three tubes, all brackish 
water G. zaddachi. 
Tube 1 : 1 specimen, a male, 15 mm long. 
Tube 2: 1 large and 1 small male, respectively 14 mm and 8 mm long. The 
large male has more spines and fewer hairs than the small one, and the 
angles of the lateral lobes of the head are acute, not obtuse as in the 
younger animal. 
Tube 3: 14 specimens, 5 — 13 mm long; nearly all females. Some of these are 
much more spinöse than the others, e. g. one large female has 4 spines 
in the lateral clusters on pleon-segments 4 and 5, and hardly any hairs 
at all, another female, the same size, has only 3 spines in the lateral 
clusters, but many more hairs developed on the antennae and peraeopods. 
Rauschen. 2 tubes labelled ”G. locusta. Rauschen. 1866“, all brackish water 
G. zaddachi. 
Tube 1*): 186 specimens, mostly small. The largest are males. None of the 
females are carrying eggs, but the presence of several young apparently 
just extruded from the marsupial pouch shows that breeding was still 
