46 STUDIES OF ANIMAL LIFE. 



"squares" (facets). Sketch a group of six 

 facets. (Use a mounted preparation, l.p.) 

 A facet is the cornea of a single simple eye. The 

 eye of the crayfish being made up of many 

 hundred such eyes is said to be compound. 

 4. The Ear-sacs. (Dem.) 



These are small sacs located on the dorsal side of 

 the basal segment of the antennules. 



H. The Respiratory System. 



Respiration is carried on by means of gills. These 

 are white feathery structures located under the 

 carapace and arranged in two groups, one on 

 each side of the thorax, in a special cavity (gill- 

 chamber). 



1. The Gill-chamber. (Examine a specimen from 



which has been removed that part of the carapace 



which is over the gills.) 

 a. Observe how far the- gill-chamber extends dor- 

 sally, ventrally, anteriorly and posteriorly. 

 h. What is the character of the inner wall ? 



2. The Gills. 



a. Observe the arrangement of the gills in sets. 



(Lift up the outer gill.) 

 h. Observe that the outer gill in each set is attached 



to the basal segment of an appendage. 

 What advantage is thus gained ? 

 c. Are the gills external or internal structures ? 



3. The Paddle. (Dem.) 



This is a thin doubly-curved plate at the anterior 

 end of the gill-chamber, belonging to the second 

 maxilla. It causes a current of water through the 

 gill-chamber. 



