98 ARTHROPODS 



moved, a thin spoon-shaped structure — the gill scoop. It 

 Hes in front of the gill chamber. Move the second maxilla 

 of the other side to see how it works. The gill scoop swing- 

 ing back and forth, pushes the water out at the front end of 

 the gill chamber. The water thus expelled is replaced by 

 fresh water, which comes up under the lower end of the 

 carapace about the bases of the legs. 



(15) The mandibles are short, hard, toothed, each bear- 

 ing a jointed appendage which lies in a groove around 

 the mandible. This is the mandibular palpus. Closely fit- 

 ting against the posterior surface of each mandible is a thin, 

 leaflike structure — the metastoma. Remove the mandible 

 and the metastoma and lay them in the series with the 

 others. Count them to see that they are all there. Remove 

 the corresponding appendages of the other side, lay them 

 in a row facing those of the side before removed, and make 

 a drawing of the series ( X2) . 



(16) The long projections in front of the head are the 

 antennae. Move them about in all directions, find the large 

 segment at the base, under the head. On this basal seg- 

 ment find a small white cone with an opening at the summit. 

 This is the aperture of the green gland in the head which 

 acts as a kidney in throwing off certain waste products of 

 the body. Remove the antenna; with the whole of the big 

 segment. What is the probable use of the bladelike branch 

 of the antenna just under the eye? Is there any similarity 

 between the antennae and the swimmerets? Are they hom- 

 ologous ? 



(17) Above the antennae are the antennulae. Remove 

 one of them and compare it with a swimmeret. 



(18) Look at the base of the antennulae for a mem- 



