56 Laboratory Guide in Zoology 



V. — Somites of the Cephalothorax. From a ventral view 

 notice how small are the sternites of the thoracic seg- 

 ments. This is caused by the enlargement of the 

 basal joints of the appendages. The unbroken sides 

 of the carapace iiepresent the united pleurites of the 

 various thoracic somites. Follow carefully the irregu- 

 lar outline of the carapace, and indications of the seg- 

 mented structure will be seen. In general it may be 

 said that each pair of limbs on the cephalothorax repre- 

 sents a single somite. 



VI. — Appendages of the Cephalothorax. Study carefully 

 the following parts : — 



a. The four posterior, nearly uniform, appendages are 

 the legs used for walking. 



b. The much larger pair, just anterior to these, are 



used for capturing and crushing food, and are called 

 chelae. 



c. Directly in front of the chelae find three pairs of 



appendages drawn closely to the body. These are 

 of the nature of legs, but since they serve to pre- 

 pare the food for the mouth, they are called 

 maxillipeds. 



d. The two appendages directly anterior to the maxilli- 

 peds are the true maxillae. Notice the scoop-like 

 structure held by the second maxilla : it is called the 

 gill scoop. 



e. In front of, and beneath the maxillse, are the hard- 



toothed jaws, or mandibles. Move them back and 



forth to see their mode of action. Is it from side 



to side, or up and down .? 

 /. The long tapering structures with large basal segments 



are the antennae : they serve as organs of touch. 

 g. The somewhat similar, but two-branched and shorter, 



