208 ANNULATA. 
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 achela 
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 profopodize, being nearest the mouth, becomes 
the gzathobase 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 large 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, ay 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 five to the endopodite. This 
completes the last leg, but the three in front of it bear 
a long hairy pad called an efipodite, and attached to its 
base is a filamentous g#//. 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 exofodite is the larger. 
