i8o THE CRUSTACEA 



thickenings of epiblast appear very early and, approaching each 

 other on the dorsal side, fuse to form an invaginated " dorsal 

 organ " (d). The young animal leaves the brood-pouch with all the 

 appendages developed. 



Advanced embryos closely similar to those of the Mysidae have 

 been observed in Lophogaster and Eucopia. 



REMARKS ON HABITS, ETC. 



The great majority of Mysidacea are marine, but a few Mysidae 

 occur in fresh water either as apparently recent immigrants from 



the sea, or as "relict" forms like the My sis 

 r dicta (Fig. 99) found in lakes in Northern 

 Europe, Ireland, and North America. A 

 few Mysidae are members of the surface 

 plankton, and a number of peculiarly 

 modified genera of that family, like all 

 .. a . the members of the other families, are 



Fio 1Q9 bathypelagic at great depths. 



Embryo of Mysis \MatromysU). Most of the Mysidae are of small size, 



a', antennule; a", antenna; d, f ew approaching Boreomvsis SCUpllOVS. which 

 dorsal organ ; /, caudal furca ; md, 3 . y . " \ ' 



mandible; th, thoracic limbs, reaches 85 mm. in length; on the other 



hand, Anchialus pusillus is only 3 mm. 



long. Among the Lophogastridae many species are of considerable 

 dimensions, and Gnathophausia ingens, the largest member of the 

 order, reaches a length of 157 mm. 



PALAEONTOLOGY. 



The genus Pygocephalus was established by Huxley in 1857 

 for a species occurring in the Coal-measures of Scotland which he 

 compared with the existing genus Mysis. Dr. H. Woodward has 

 recently made the highly important discovery that Pygocephalus 

 possessed a brood-pouch formed by six or seven pairs of imbricating 

 oostegites (Fig. 110), thus showing that it must be classed with 

 the Peracarida. Pygocephalus has a broad and apparently flattened 

 b6dy, with a carapace covering the thoracic region. The antennules 

 are biramous and the antennae have a broad scale -like exopodite. 

 Seven pairs of thoracic limbs are visible in the fossils (the first pair 

 were probably folded inwards as maxillipeds and are therefore 

 invisible), carrying each a multiarticulate exopodite. The uropods 

 and telson form a broad tail-fan. This combination of caridoid and 

 Peracaridan characters justifies us in assigning Pygocephalus a place 

 among the Mysidacea, although it is impossible at present to define 

 more precisely its relation to the existing families of the order. 

 It is not known whether the tergal portions of any of the thoracic 



