THE GILLS. 151 



or coxopodite, being very broad, and projecting 

 only very slightly from the surface of the head : 

 on its ventral surface is a tubercle perforated 

 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 setae, 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 from 

 the sides of the thorax and from its appendages. Primi- 

 tively 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 branchiostegite. 



Expose the left branchial chamber by cutting away the 



