64 
LABORATORY EXERCISES 
gills. How does this explain what you have seen? Think of 
experiments which you might try in order to test the taste, smell, 
hearing, sight, and feehng of this animal. Recall observations 
which you have previously made regarding the habitat and ac¬ 
tivities of the crayfish. Watch them whenever the opportunity 
is afforded you while on field trips, fishing trips, etc. 
II. External Structure. 
1. The crayfish belongs to the same great group of animals as 
the insects. In the crayfish we have an example of high specializa¬ 
tion. This is shown in the widely varying functions of many 
separate structures of similar origin. The appendages, which 
show such specialization, are of similar origin. If they are ex¬ 
amined closely, a marked hkeness in number and arrangement of 
segments is found. Structures of similar origin and general plan 
are known as homologous structures. Where is the skeleton of the 
crayfish? It is called the exoskeleton. What is the nature of this 
covering? How is it formed? How does molting enable this 
animal to accommodate its covering to its size? The body of the 
crayfish is divided into the cephalothorax and the abdomen. How 
many segments are there in the abdomen? The crayfish’s body 
is divided into twenty-one segments. Locate as many of these as 
possible. With the exception of the first and last, each segment 
bears a pair of appendages. 
2. The Cephalothorax. The cephalothorax is divided by a 
tran'sverse groove into two regions — the head and the thorax. 
The exoskeleton of the dorsal and lateral surfaces of the cephalo¬ 
thorax is known as the carapace. Begin your study at the an¬ 
terior end. Note the sharp beak, the rostrum, which lies between 
the eye-stalks. What are the size and position of the eyes and how 
are they attached? Why are the eyes stalked ”? Behind the 
eyes are the antennules, slender double feelers. At the base of 
these find the statocysts. (These are small, sac-like sensory organs.) 
Back of the antennules are the antennce. At the base of these are 
the openings of the excretory organs, the green glands. The jaws, 
the mandibles, are next in order. In what direction do they move? 
