HEAD APPENDAGES 133 



behind the mandible is a small uujointed plate, leaf- 

 like in form. It is not regarded as a true appendage, 

 but as an outgrowth of the metastoma, which forms 

 the posterior and lateral borders of the mouth. 



II. The antenna consists of protopodite, exopodite, and 

 endopodite, the last of which forms the long 

 ' feeler.' 



i. The protopodite consists of two joints ; the first, 

 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 

 joint, or basipodite, is also short and stout, and 

 bears the endopodite and exopodite. 



ii. The endopodite has two stout basal joints and a 

 long slender many-jointed feeler. 



iii. The exopodite is a very stout sharp-pointed 

 blade, the scaphocerite. 



I; The antennule is smaller than the antenna. 



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

 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 setas, which 

 extend across it from its outer Up. 



ii. The endopodite and exopodite are two many- 

 jointed filaments, the exopodite being sUghtly 

 the larger. 



D. The Gills and Gill-chambers. 



The giUs are highly vascular external outgrowths from 

 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. 



