THE AMPHIPODA 239 



be noticed ; the occurrence of si)ecies from geneni otherwise marine 

 in the "relict" faunas of lakes in Northern Eur()i)e and America 

 and in the Caspian Sea ; the peculiar blind subterranean species 

 {Niphargus, etc.) which come to the surface in wells and penetrate 

 into the abyssal waters of deep lakes ; and the radiation of single 

 genera into numerous species in a limited area in the Gavimari of 

 Lake Baikal and the HijahUae of Lake Titicaca. The only terres- 

 trial Amphipoda occur among the Talitridae, which in northern 

 latitudes live, for the most part, just al)0ve tide-marks, but in the 

 warmer regions of the globe penetrate inland to great distances. 



Perhaps no Amphipoda except the whale-lice (Cyamidae) (Fig. 

 135) are truly parasitic, though some forms with suctorial mouth- 

 parts seem to be semi-parasitic on fish {7'rischizostoma). Many 

 species, however, are commensal with sponges and other organisms. 

 The pelagic Phronima (Fig. 136) lives in a transparent barrel-shaped 

 case fashioned from the swimming-bell of a 8iphonophore or from 

 a test of a pelagic Tunicate. 



The Gammaridea probably include the smallest as Avell as the 

 largest Amphipoda, for many species do not exceed two or three 

 millimetres in length. The lai-gest is Alicella gigantea, Chevreux, 

 which reaches a lenirth of 140 mm. 



Palaeontology. 



Fossil remains of Am})hipoda are exceedingly rare, and although 

 various problematical fossils from Palaeozoic rocks have been 

 referred to this group, it is only in the Tertiary and Post-Tertiary 

 deposits that undoubted Amphipoda have. been found. All l)elong 

 to the Gammaridea, and though a genus Palaeoganunarus has been 

 established for a species found in Baltic amber, its generic and even 

 specific distinctness from some of the living forms of Gammarus is 

 uncertain. 



Affinities and Classification. 



Although the Amphipoda plaiidy beloTig to the Peracaridan 

 division of the Malacostraca, their relation to the other Orders of 

 the division is by no means so clear as in the case of the Isopoda. 

 It seems very likely that their affinity to the Isopoda is not so close 

 as has been supposed. Apart from the sessile eyes and the 

 segmentation of the body, characters which, there is reason to 

 suppose, have originated independently in at least one other case 

 (Koominga), almost the only point of agreement between the two 

 Orders is found in the possession of coxal plates on the thoracic 

 somites. But coxal plates are not developed in the most primitive 

 Isopoda, the Asellota, and if their appearance in that Order was 

 later than the acquisition of the typical Isopodan characters, the 



