X INTRODUCTION. 



rant form by the test of the known typical characters, 

 might find it impossible to refer it to its true relations, 

 without an investigation of the intermediate affinities, and 

 an acquaintance with the laws which regulate their 

 variations. 



The separation of the true EPIZOA from the Crustacea 

 has indeed, in some measure, facilitated the arrangement 

 of the latter class, and enabled the zoologist to restrict 

 within intelligible limits the characters which belong to 

 the group. 



I shall therefore, in the following sketch, consider the 

 CRUSTACEA, the EPIZOA, and the CIRRIPEDES, as constituting 

 three distinct types of form ; with this restriction the 

 Crustacea may be defined as articulated animals, having 

 each segment of the external skeleton furnished with ar- 

 ticulated appendages ; they are all of them free or loco- 

 motive ; the respiration is branchial, and they are, with 

 very few exceptions, arpaatic in their habits ; the circula- 

 tion is carried on by means of a complete vascular system, 

 and is of a mixed character, the blood being received into 

 an aortic heart, both from the branchiae and from the 

 system, and circulated in a mixed or partially decarbo- 

 nized condition. The nervous system resembles, in its 

 general principles, that of the Insects. It is gaugliouic, 

 longitudinal, and generally distinctly developed. The 

 sexes are separate. 



Such are the general characters by which the Crustacea 

 proper may be distinguished, and which appear to be 

 sufficiently defined, as far as our present knowledge ex- 

 tends. A further insight into the structure of each system 

 of organs, as existing in the different orders and families 

 of the class, will show how various and startling are some 

 of their modifications. 



