xlviii INTRODUCTION. 



a well-developed ganglion), the seventh segment of the 

 pereion to have its ganglion consolidated with those that 

 supply the caudal region a view that our own observations 

 lead us to believe has been founded on a misconception. 



GEOGRAPHICAL DISTRIBUTION. 



The Geographical Distribution of the two orders of the 

 Sessile-eyed Crustacea, if made under careful and extensive 

 observations, would (from the great amount of the modifi- 

 cation of parts, while a close assimilation of general form is 

 very persistent throughout great numbers of genera) afford 

 one of the most interesting and, we believe, instructive 

 chapters in the distribution of life over the globe. 



The subject has not yet sufficiently been worked out 

 so as to approximate to correct information ; for so much 

 of the earth's surface has yet to be searched, that it is by 

 no means improbable that new arid intermediate forms 

 may frequently be found in places that are yet" unknown, 

 so that forms that as yet are described as species or genera 

 may be only modified forms of one species, or, as has been 

 demonstrated by M. Hesse with respect to Anceus and Pra- 

 niza, that animals placed by authors in separate genera 

 and in distinct families may be only sexually distinct. 



Such imperfect information as is at our command, 

 while it does not enable us to grasp the subject so as to 

 do justice to it as a whole, has yet enabled us to observe 

 some points of interest that our British species possess in 

 relation to exotic forms. 



With the exception of a single specimen, brought from 

 Algiers by M. Lucas, the genus Talitrus is only known as an 

 inhabitant of the northern and western coasts of Europe, 

 while its closely allied form, Orchestia, and its congeners, 

 excepting Nicea, of which we know but one or two species 

 (which tend to corroborate the assertion), appears to be 



