2o8 ANNUL AT A. 



sea-floor became the vogue, the swimmerets in the neigh- 

 bourhood of the centre of gravity became modified for bear- 

 ing the weight of the body. In this case the endopodites 

 evidently form the main axis of support, being nearest the 

 perpendicular through the centre of gravity, and the 

 exopodites, being superfluous, disappear. A *'leg" or a chela 

 therefore consists of protopodite and especially endopodite. 

 Both parts become jointed for further movement, so the 

 protopodite acquires two sclerites and the endopodite five. 



On the other hand, the appendages near the mouth 

 naturally take part in the ingestion of food. In this the 

 basal part or protopodite^ being nearest the mouth, becomes 

 the gnathobase or jaw-element. Hence the jaw-elements 

 always consist largely of protopodite^ the endopodite and 

 exopodite becoming subsidiary. These three axioms should 

 be held in mind : — 



1 . Swimming organs at the hind end, retaining their prim- 

 ary functions, retain the primitive biramous condition with 

 equal endopodite and exopodite. '^'' 



2. Walking organs, round the centre of gravity, lose the 

 exopodite and have a large and complex endopodite. 



3. Eating organs, round the mouth, lose the exopodite 

 and reduce the endopodite, but have a \zxgQ protopodite. 



4. Tactile organs, at the anterior end, may be specially 

 modified, with or without reduction. 



5. Organs between these, with ill-defined functions, such 

 as foot-jaws, may retain all the parts more or less modified. 



(It will be found convenient to remove the appendages 

 from the last leg forwards as the jaws overlie each other 

 forwards.) 



If the four legs be removed we can at once contrast them. 

 The two first have pincers at their ends, or are chelate, 

 whereas the two last are non-chelate. Each has two joints 

 to the protopodite and fvvQ to the endopodite. This 

 completes the last leg, but the three in front of it bear 

 a long hairy pad called an epipodite, and attached to its 

 base is a filamentous gill. If the specimen be a male, 

 the genital aperture will be found on the basal joint of 

 the last leg, whereas, if a female, the genital aperture 



* In a few cases, as in the sixth abdominal, the exopodite is the larger. 



