io4 
ANIMAL ACTIVITIES. 
Fig. 90. — Fourth Ab¬ 
dominal Segment of 
Crayfish. /, tergum; 
st , sternum; //, pleu- 
rum; /r, protopodite; 
en, endopodite; ex , ex- 
opodite. One append¬ 
age has been removed. 
Notice the form of one of the swimmerets on the 
second or third abdominal segments and compare all 
the swimmerets with this. 
How do the other swimmerets 
differ from the ones first exam¬ 
ined ? 
Infer a use for the sixth pair of 
swimmerets. 
Sketch a swimmeret, naming 
parts. 
Compare the ends of the legs 
having pincers with those which 
do not have those organs. How 
much extra growth would be 
needed to produce a pair of 
pincers on the last pair of legs ? 
Examine all the jointed ap¬ 
pendages on one side of the 
body. How many do you find ? Counting a pair of 
appendages to each segment or somite y how many 
segments has a crayfish or shrimp ? 
How do the mouth-parts of a crayfish or shrimp differ 
from those of a grasshopper ? 
Remove one side of the carapace and expose the 
gill-cavity. Do you find gills at the base of all the 
legs ? Do you see the spoon-shaped gill-scoop ? Of 
what appendage is it a part ? 
Look on the inside of the basal joints of the legs to 
find the outlets of the reproductive organs. Among 
crayfish the females have these openings on the middle 
pair of legs and the males on the last pair. 
With the help of the drawings find the openings of 
the green glands (renal openings). 
Find the ear. 
Where is the anal opening ? 
Internal Structure. With an alcoholic specimen 
one can make out the parts indicated in the figure 
below. The position of the heart, digestive tube, and 
