A CRAYFISH OR A LOBSTER 29 
segmentation, because of the fusion of the somites and the 
presence of a hard, shield-like structure covering it, which is 
called the carapace, but on the ventral side the segmentation is 
distinctly seen. Extending along the entire ventral surface 
of the animal are the paired appendages. Their metameric 
significance may not be seen in the cephalothorax, but it will 
be distinctly seen in the abdomen, where each somite except 
the last bears a pair of appendages. 
The cuticular exoskeleton is thicker and heavier than in 
insects ; this is due to the presence, besides chitin, of salts 
of lime. The crayfish or lobster moults its cuticula periodi- 
cally, the adult animal probably once or twice a year, the 
young animals oftener. 
The animal is capable of two sorts of locomotion. By pow- 
erful strokes of the broad, fin-like end of the abdomen it swims 
rapidly backward, and it can walk on its thoracic legs. It is 
well provided with special sense-organs. Most important to it 
are the two pairs of feelers or antenne, which are characteristic 
of all crustaceans, and the compound eyes on movable stalks. It 
also possesses, in a pair of small cavities, on the upper surface 
of the basal joints of the first or shorter pair of antenna, pecu- 
liar, sense-organs, which were formerly supposed to be ears, but 
are now known to be balancing organs. With the aid of them 
the animal maintains its equilibrium. 
The body of the crayfish or the lobster, as of all the higher 
crustaceans, is made up of twenty somites or body-segments, of 
which the thirteen anterior somites form the cephalothorax, 
and the seven posterior ones the abdomen. 
The cephalothorax. The anterior five somites forming this 
body-division are cephalic, the remaining eight are thoracic, 
and all are covered dorsally and laterally by the carapace. The 
projection running forward from the anterior end of the 
carapace is called the rostrum. A transverse groove is seen 
near its middle; this is the cervical suture and marks the 
