HOMOLOGIES AMONG CRUSTACEA. 107 
Serial Homology. In comparing the jointed appen- 
dages of the different segments of the abdomen of a 
lobster or crayfish with one another, 
we note the fact that each is com- 
posed of a basal joint of two seg- 
ments and a pair of jointed branches. 
One part of the basal joint is called 
the Jdasipodtte and the other the cox- 
opodite; both together are called the 
protopodite. The inner branch is 
the endopodite, and the outer branch 
the exopodite. In the other jointed 
appendages we find striking similarity 
to the swimmerets. They are also 
similar in origin; starting as bud-like 
outgrowths from the rings or somites 
of the embryo. Indeed, each seg- 
ment is homologous with every 
other. This kind of homology is 
called serial homology. It is very 
noticeable among the crustacea. 
Laboratory Exercise. Examine 
a sand-hopper and an asellus. 
How does each compare with the 
shrimp in regard to: 
(a) The number of legs? 
Fic. 93.—Walking 
Appendage of 
Crayfish with Gill, 
&, Attached. 
(6) The number and form of swimmerets ? 
(c) The number of antennz ? 
(2) The number of segments? 
(¢) The divisions of the body? 
(7) The eyes? 
(g) The carapace ? 
(Z) The position of the gills ? 
(¢) The general form of the body? 
Name parts of the sand-hopper which are homol- 
ogous with those in the shrimp. 
Name an organ in the sand-hopper which is anal- 
ogous to one in the shrimp but not homologous with it. 
