INTRODUCTION. xxi 



Mysidce, discovered by Mr. Norman. The telson in the 

 Sessile-eyed Crustacea is generally an abortive, and fre- 

 quently a rudimentary, part. In the Isopoda, 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 chelte 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. 



