xlvili INTRODUCTION.* 
a well-developed ganglion), the seventh segment of the 
pereion to have its ganglion consolidated with those that 
supply the caudai 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 and 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 
