186 ANIMALS WITH JOINTED LEGS— THE ARTHROPODS 



head and the next eight to form the thorax. The head and thorax are 

 so closely connected that they form a single part of the body, the cephalo- 

 thorax. The final six segments form the abdomen. Thus, there are 

 eighteen segments to the whole body ; but, since there is an additional 

 pair of appendages, the antennules, which are apparently not associated 

 with a segment, there is a total of nineteen pairs of appendages. The 

 true appendages in the embryo are all of the typically biramous type 

 consisting of a basal portion attached to the body, the protopodite, which 

 branches into an outer branch, the exopodite, and an inner branch, the 

 endopodite. As development proceeds, however, the appendages are 

 modified in various ways, each appendage becoming adapted to perform 

 a particular function. In the adult stage they exhibit a great variety 

 of shapes and sizes due to these modifications. Some of the branches 

 may be greatly reduced or even lacking entirely, and some appendages 

 have developed an entirely new attachment, the epipodite, which projects 

 dorsally from the protopodite. The accompanying drawings illustrate 

 the diversity in structure that exists between the different appendages. 

 A brief listing of their uses will illustrate their diversity of function. 



Antennule — two jointed filaments ; touch, taste, equilibrium. This is 

 not a typical appendage and does not represent a segment according to 

 the latest research. 



1. Antenna — a jointed filament with excretory pore in basal seg- 

 ment ; touch, taste. 



2. Mandibles — crushing food. 



3. First maxilla — small and delicate ; helps guide food to the mouth. 



4. Second maxilla — modified into a bailer, epipodite present. 



5. First maxilliped — exopodite, a many jointed filament, epipodite 

 present; taste, touch, holds food. 



6. Second maxilliped — bears a gill, endopodite coarse, exopodite 

 slender; touch, taste, holds food. 



7. Third maxilliped — bears a gill, endopodite much larger than ex- 

 opodite ; touch, taste, holds food. 



8. First walking leg, or cheliped — bears a gill, exopodite absent, 

 distal two joints form a powerful pincher ; catching animals, cutting 

 them up, and defense against enemies. 



9. Second walking leg — bears a gill, exopodite absent, walking and 

 grasping with its small pinchers. 



10. Third walking leg — same as second leg. 



11. Fourth walking leg — same as third leg, but pinchers are absent. 



12. Fifth walking leg — same as fourth leg, but may be used to clean 

 abdomen. 



