118 CRUSTACEANS, 



forward, out of the chamber. Watching it closely, pull 

 the maxilla of the opposite side backward and forward, 

 and you will see how it acts. Because it keeps the water 

 moving, it is a very important accessory organ of respira- 

 tion. The water that bathes the gills must continually be 

 renewed, in order that sufficient oxygen may be obtained 

 from it for the needs of the body. 



In front of the maxillae, find a pair of short, hard, 

 toothed mandibles, each with a small, three- jointed palpus 

 lying in a groove on its anterior surface. Study the action 

 of the mandibles. Find a thin, leaf -like, un jointed plate 

 (the metastoma), fitting closely against the posterior sur- 

 face of the mandible. This is not considered as a true 

 serial appendage, but only as an outgrowth from the 

 border of the mouth. 



On the front of the head are two very long antennas. 

 Draw one of these downward, and find a large, basal seg- 

 ment, bearing on one side a light-colored, conical process. 

 In this process the duct from an excretory gland in the 

 head terminates. Find, also, a blade-like branch of the 

 antenna beneath the eye. What is it there for ? 



The evidently two-forked, short appendages above the 

 antennae are the antennules. Beneath the eye, in the base 

 of each antennule, is a so-called ear sac. 



Compare in structure an antennule with an antenna ; 

 with a mandible ; with a maxilla ; with a maxilliped ; 

 with a leg ; with a swimmeret. Observe that they all 

 exhibit modifications of one type of structure ; and the type 

 is a two-jointed pedicel, bearing (normally) a pair of 

 jointed branches. In the legs, one of the branches is 

 suppressed, and the other is enormously developed to form 

 the part of the leg comprised in the five terminal segments. 

 In all the thoracic appendages, except the first and the 

 last, there is an internal process from the basal segment, 

 forming a gill. 



