Crustacea. 39 



11. Anterior to the big legs are several pairs of appendages 

 surrounding the mouth. Probe between them to find the mouth. 

 These mouth parts are numbered from the front, but on account 

 of the way in which they overlap, it is easier to remove and study 

 them in the reverse order. 



12. The appendages just in front of the big claws are the hind- 

 most of three pairs of jaw feet, or maxillipeds. Gently raise them 

 to see how they cover the other mouth parts. Note that these 

 maxillipeds, or foot jaws, have an inner branch (endopod), which 

 meets the corresponding part of the opposite maxilliped, and an 

 outer branch (exopod). Observe that both these branches are 

 attached to one segment (protopod), next to the body. Seize this 

 basal segment, and remove the whole maxilliped. Compare it 

 with one of the swimmerets of the third ring of the abdomen. In 

 the same way remove the second and first maxillipeds of this side, 

 keeping them in order. Are there gills attached to the maxilli- 

 peds? Is there more than one gill on each leg? Are there other 

 gills than those attached to the legs? Pick one of the gills to 

 pieces under water to determine its structure. After removing the 

 gills, look in this region for further traces of thoracic rings. 



13. Anterior to the maxillipeds are two pairs of maxillae. These 

 are very thin, and lie close to each other, so that if great care be 

 not taken, they are likely to be pulled off together. Investigate 

 closely, 'and then, inserting the forceps well down, remove them, 

 one at a time. Attached to the base of the hinder maxilla is a thin, 

 double spoon-shaped structure, the gill scoop, or gill paddle. It 

 lies in the front part of the cavity in which the gills are, the gill 

 chamber. With the forceps move back and forth the second 

 maxilla of the other side, to see how the gill scoop is thereby 

 moved. The gill scoop, swinging back and forth, pushes the 

 water out of the front end of the gill chamber. The water thus 

 expelled is replaced by fresh water, which comes up under the 

 lower edge of the carapace, about the bases of the legs ; thus the 

 gills are constantly bathed with water containing a fresh supply of 

 oxygen. 



