82 CRUSTACEA. 



the existence of the great affinity between certain of the 

 least perfectly organized fishes, and the most perfectly organ- 

 i/ed mollusca, as the Cephalopoda. 



The following are the characters by which the animals of 

 this class are distinguished from the other annulosa. They 

 are destitute of internal vertehrsr, having white blood ; the 

 body divided into various segments (differing in number), 

 incased in a crustaceous envelope, and provided with articu- 

 lated legs ; respiring by means of branchiae or branchial 

 plates, ordinarily annexed to the base of the legs or lower 

 jaws ; having a distinct heart furnished with visible vessels, 

 with legs, of which the number is generally five or seven 

 pairs, and always destitute of wings ; head generally con- 

 founded with the thorax, furnished with two pairs of an- 

 tennae, and a pair of mandibles (often palpigerous) ; two 

 pairs of lower jaws, and three pairs of foot-jaws (the two 

 posterior pairs of which are transformed into two additional 

 pairs of legs, when there are seven pairs of the latter or- 

 gans) ; mouth also furnished with an upper and lower lip, 

 or rather a tongue, the external pair of foot-jaws performing 

 the office of a lower lip ; eyes two, facetted, often borne 

 upon footstalks ; sexes distinct ; sexual organs placed either 

 at the base of the legs or the extremity of the body. 



And here may be noticed the great difficulty existing in 

 the determination of the nomenclature of the different or- 

 gans in the various groups of annulose animals, some of 

 which recede so entirely from those which, by common con- 

 sent, are considered as the types or normal divisions of the 

 class; and, indeed, the remark is applicable in every other 

 branch of the creation. I mention this subject, because 

 Dr. Johnston, the celebrated naturalist of Herwick-upon- 

 Tweed, has recentK published some observations bearing 

 upon this view of the question, in the Magazine of Natural 

 History, which deserve attention. Speaking of one of the 



