FOR WAGGONER’S BIOLOGY 
65 
Experiment to find out. The next appendages are the maxillce. 
What is their function? The remaining appendages of the cephalo- 
thorax belong to the thorax proper. Find the three pairs of 
appendages, the maxillipeds, which are next to the maxillae. 
What is their function? The five pairs of appendages posterior 
to the maxillipeds are the legs. Study the legs which bear large 
claws. How many joints are present in these appendages? In 
what directions can they move? For what are they adapted? 
Compare the remainder of the thoracic appendages with these. 
How many pairs are there? What is the function of each? 
3. Abdomen. Examine the middle segment of the abdomen. 
How is it attached to the others? Of what advantage is such an 
attachment? In what direction can this segment move? Note the 
appendages which are attached to it. These are called swim- 
merets. How many are there in each segment and what is their use? 
How many segments have such appendages? Compare all the 
segments of the abdomen, and compare their appendages as to 
structure and to use. What is the special function of the last two 
segments and of the appendage which the next to the last segment 
bears? The last segment is the telson. 
4. List all the external structures adapted for locomotion, for 
food getting, and for the receiving of sensations. Draw the side 
view of the crayfish. Draw one of the third pair of walking legs 
and one of the swimmerets. 
III. The Internal Structure. Study a prepared dissection of a 
crayfish in which the dorsal wall of the abdomen and thorax has 
been removed. Note the heart, which is in the median line of the 
cephalothorax. It lies in a cavity which receives the arterial blood 
from the gills. This blood enters the heart through three pairs of 
valves that open inwards. From the heart five blood vessels pass 
anteriorly and another passes posteriorly to the dorsal part of the 
abdomen. From the latter a branch is sent out to the ventral 
region. The circulatory system is an open one; for in the body 
tissues the blood leaves the vessels, and it is returned to the heart 
through a series of open spaces. Remove the side of the shell just 
