INTRODUCTION. XX1 
Myside, discovered by Mr. Norman. The telson in the 
Sessile-eyed Crustacea is generally an abortive, and fre- 
quently a rudimentary, part. In the Jsopoda, except in 
the genera Apseudes and Anthura, it is always fused with 
the preceding segment. 
The composition of the dermal skeleton is, in all Crus- 
tacea, the same. In the Sessile-eyed order the texture is 
very thin, and seldom consolidated into a firm structure, 
except in certain parts of some few genera where strength 
is required, as in the chele of large-handed species. This 
circumstance offers the advantages of enabling the 
observer to examine the internal structure of the animal 
without the necessity of dissection. During the life of 
the animal, we are enabled to trace the currents of circula- 
tion of the blood, the motion of the cardiac vessel, and the 
position of the internal organs in relation to each other. 
This delicacy of the structure also enables us to dis- 
cover the very diverse and varied arrangement of the 
material of which it is built up, and demonstrates (con- 
trary to our anticipations) that in species often closely 
allied, there is a very distinct appearance in the micro- 
scopic structure. It may prove to be of some importance 
in determining species, but care should be taken that the 
several specimens examined should be taken from the 
same part of the skin of each animal. We have illus- 
trated many of these varieties of structure throughout the 
work, in connection with the animals to which each 
belongs. 
Frequently, besides the markings that illustrate the 
manner in which the skin is built up, there is another 
that is not always constant, consisting of a series of small 
perforations through the tissue, which in some species 
assume a waved appearance, as may be observed in the 
genus Ampelisca. 
