40 Practical Zoology. 



14. The mandibles are short, hard, and toothed. Each man- 

 dible bears a jointed appendage, the mandibular palp, which 

 curves around the anterior edge of the mandible in a groove. 

 Move a mandible about to see how it is hinged. 



15. Closely fitting against the posterior surface of each mandi- 

 ble is a thin, leaflike structure, the metastoma. The metastoma 

 differs from the maxilla in pointing outward and in being un- 

 divided. Remove it and complete the series of mouth parts, 

 mandible, first maxilla, second maxilla, first maxilliped, second 

 maxilliped, third maxilliped. Remove the corresponding append- 

 ages of the other side, lay them in a row facing those of the oppo- 

 site side as before removal, but not now overlapping each other, 

 and make a drawing of the series, naming them. 



1 6. The long projections in front of the head are the antennae. 

 Seize one of them with the forceps, and pull about in all directions, 

 to make out the large segment, at its base, under the head. On 

 this basal segment find a small white cone, with a hole at its sum- 

 mit. This is the aperture of the kidney, or green gland. Remove 

 the antenna, with the whole of this big segment at its base. What, 

 probably, is the use of the bladelike branch of the antenna 

 just under the eye? Compare the antenna and its branches with 

 a swimmeret. 



17. Above the antennae are the antennules. 



1 8. In the base of each antennule, just underneath the eye, is 

 the ear sac. 



19. With the forceps pull the eye about to see its range of 

 motion. Pull it out by its stalk, and examine with lens and micro- 

 scope its black tip, or cornea. Each distinct area is a facet, and 

 the eye is compound. 



20. After removing the cephalothoracic appendages, and the 

 carapace, carefully clean and thoroly examine the framework 

 of the cephalothorax, still looking for traces of thoracic rings. 



21. The skeleton of a crayfish, like that of insects, is an external 

 skeleton, or exo-skeleton. Compare it with the internal skeleton, or 

 endo-skeleton, of vertebrates. 



