318 A HISTOKY OF RECENT CEUSTACEA 



three and generally the first five segments of the pleoa are 

 short, whether fused or articulated ; the sixth comprising 

 the telson is usually the largest, and its appendages con- 

 stitute the only pair of uropods. The pleopods when 

 present are almost always closely overlapping. 



Instead of being contented with the single genus 

 Oniscus^ as in the first edition of Linnaeus's ' System of 

 Nature,' the Isopoda now occupy seven tribes, several of 

 which are of great extent. These are named respectively 

 Chelifera, Flabellifera, Valvifera, Asellota, Phreatoicidea, 

 Epicaridea, and Oniscoidea. 



Tribe \. --Chelifera. 



This first tribe, the ' claw-bearing ' Isopods, is a com- 

 paratively small one, but it has many peculiarities and is 

 controversially interesting. It has to do with narrow, 

 subdepressed, or subcylindrical animals, in which the head 

 is united with the first or even the first and second seg- 

 ments of the perseon. The segments of the pleon are 

 sometimes fused together, and in that case the pleopods 

 are wanting. Otherwise these appendages are generally 

 present, and consist of a two-jointed stem and two un- 

 jointed branches, but differing from those of the Isopoda 

 in general in being swimming rather than branchial 

 organs. In this tribe respiration is carried on by means 

 of a branchial chamber situated under the sides of the 

 carapace to the rear. The first maxilla have a backward- 

 directed ' palp ' similar to that described in the Cumacea, 

 and no doubt fulfilling the same function of cleausing the 

 branchial chamber from obstructions. It is supposed to 

 be a part of the endopod turned backwards. The maxilli- 

 peds have also a remarkable structure directed backward 

 into the branchial chamber. This by its attachment to 

 the first joint is recognised as the epipod. By its rhyth- 

 mical movement to and fro within the cavity it maintains 

 a constant influx of water for the oxygenation of the 

 animal's blood. In evident connection with this respira- 

 tory process in the front part of the body is the circum- 



