A CRAYFISH OR A LOBSTER 23 



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 e*ntire 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 posterior end of the abdomen 

 it swims rapidly, 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 antennae, 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 antennse, 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 



