SOME BRACKISU-WATER AMPHIPODA. 657 



described below, cei-tain marked modifications occur which cannot 

 be referred to either of these influences. They would appear 

 to be caused by the animal's environment, according as it lives 

 in fresh water or in salt. The difference in appearance between 

 a typicfJ adult freshwater specimen and a typical marine or 

 brackish-water one is so extreme as to suggest their belonging 

 to distinct species, but structurally they are identical, and 

 intermediate forms are very common (see below, " Moorflether 

 Concave"), apparently varying with the degree of salinity, though 

 on this point I cannot as yet speak definitely. Experiments 

 have been instituted at the Laboratory here with an allied species 

 to try and determine the question of the effect of salinity on 

 the animal and the length of time for such effect to become 

 evident. 



Gammarus zaddachi, sp. n. (Pis. LXXIII. & LXXIY. 

 figs. 1-12.) 



= 1844, GammanfjS locusta Fabr. ?, Zaddach, Syn. Crust. Pruss. 

 p. 4. 



= 1878. Gammarus locii,sta'F{ihv.,Z&MsuGh, Die Meeres Fauna 

 an der preussischen Kiiste, pp. 26-32. 



= 1886. Gammarus index Kraepelin, Die Fauna der Hamburger 

 Wasserleitung. Abhand. Geb. d. Naturw. Verein in Hamburg, 

 Bd. ix. H. 1. 



= 1907. Gammarus pulex^oVK, Mitteil. biol. Elbeuntersuch- 

 ung. ISTatui-h. Museums in Hamburg. Verhandl. Naturw. Yer. 

 Hamburg. 



= 1911. Gammarus locusta L., Yanhoffen, Beitrage z. Kennt. 

 d. Brack wasserfauna im Frischen Haff. Sitzung. d. Gesellsch. 

 naturf. Freunde, Berlin, 1911, no. 9. 



It is with reluctance that I have felt myself obliged to institute 

 a new species in the already overci-owded and confused genus 

 Gammarus. The species of Gammaribs are difficult to separate 

 except by the secondary sexual characters of the adult males; 

 the immature of all the species are practically indistinguishable 

 from each other, and even the females are not easy to differen- 

 tiate. The species now under discussion has been frequently 

 confounded with others, the freshwater form with G. pidex, and 

 the marine and brackish- water form with G. locusta and G. d%ie- 

 henii; but it can be distinguished from them by a glance at the 

 antennas, both of which are characterised by clusters of long out- 

 standing hairs, and by the form of the 4th sideplate and the 

 3rd uropod. 



The brackish-water specimens are characterised by their slender- 

 ness and transjjarency, and the tenuity of their epidermis, while 

 the freshwater ones are broad, very robustly built, the epidermis 

 thick, strong and opaque, the basal joints of the hinder perseo- 

 pods narrower especially in the old males, and with a much 

 denser supply of the long fine hairs developed on the antenna, 

 the peragopods, particularly the hinder ones, the pleon, the 3rd 



