SOME BRACKISH-WATER AMPHIPOD> . 657 



clesCTibed below, certain 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 difierence in appearance between 

 a typical 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 institxited at the Laboratory here with an allied species 

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

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

 evident. 



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

 figs. 1-12.) 



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

 p. 4. 



= 1878. Gammarus locusta Fabr., Zaddach, Die Meeres Fauna 

 an der preussischen Kiiste, pp. 26-32. 



= 1886. Gammarus pulex Kraepelin, Die Fauna der Hamburger 

 Wassex'leitung. Abhand, Geb. d. JSTatarw. Verein in Hamburg, 

 Bd. ix. H. 1. 



= 1907. Gamma7'its pulex Yolk, Mitteil. biol. Elbeuntersuch- 

 ung. I^aturh. Museums in Hamburg. Verhandl. Naturw. Yer. 

 Hamburg. 



= 19ll. Gammarus locusta L., Vanhoffen, Beitrage z. Kennt. 

 d. Brackwasserfauna im Frischen HaflT. 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 overcrowded and confused genus 

 Gam,marus. The species of Gatumarxis 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. pulex, and 

 the marine and brackish- water form with G. locusta and G. due- 

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

 antennae, both of Avhich 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 transparency, 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 antennae, 

 the peraeopods, particularly the hinder ones, the pleon, the 3rci 



