THE GILLS. 143 



posteriorly by the renal aperture, The second 

 segment, or basipodite, is also short and stout, 

 and bears the endopodite and exopodite. 



ii. The endopodite has three stout basal segments and 

 a long slender many-jointed feeler. The first 

 segment is divided longitudinally into an inner 

 and an outer part, movable on each other, and 

 thus allowing rotation of the next segment. 



iii. The exopodite is a stout sharp-pointed blade, 

 the scaphocerite. 



I. The antennule is smaller than the antenna. 



i. The protopodite is three-jointed, the basal segment 

 being much the largest, and containing the 

 ' auditory ' organ, which opens on its dorsal 

 surface. The aperture is three-cornered, and 

 is guarded by a series of plumose setse, which 

 extend across' it from its outer lip. 



ii. The endopodite and exopodite are two many- 

 jointed filaments, the exopodite being slightly 

 the larger. 



D. The Gills and Gill-chambers. 



The gills are highly vascular external outgrowths froni 

 the sides of the thorax and from its appendages. In them 

 respiration occurs, this process consisting in an interchange, 

 by diffusion through the thin cuticle, of the gases dissolved 

 in the water and in the blood. The gills are enclosed and 

 protected by downward growths of the carapace at the sides 

 of the thorax, the branchiostegites. 



1 . The branchial chambers are situated one at each side of 

 the thorax, between the body and the branehiostegite. 



Expose the left branchial chamber by cutting aivay the 

 branehiostegite of that side ; and fix the animal on its right 

 side under water. 



The chamber is bounded on its inner side by 

 the thoracic wall, and on its outer side by the 



