CRAYFISH, LOBSTERS, CRABS, SHRIMPS, ETC. 107 



ganglia, arranged segmentally. The two ganglia of each pair 

 are fused more or less nearly completely to form a single gang- 

 lion, and the nerve-cords are partially fused, or at least lie 

 close together. In addition there is a smaller sympathetic 

 system composed of a few small ganglia and certain nerves 

 running from them to the viscera, this system being connected 

 with the main or central nervous system. In this group the 

 organs of special sense reach for the first time a high stage 

 of development. Compound eyes are peculiar to Arthropoda. 

 The heart lies above the alimentary canaK Respiration is 

 carried on by gills, in the aquatic forms, and by a remarkable 

 system of air- tubes or tracheae in the land forms (insects). 

 The sexes are usually distinct, and reproduction is almost uni- 

 versally sexual. Most of the species lay eggs. 



CRUSTACEANS 



The members of the large and important class Crustacea 

 are mostly aquatic, but a few species are found on land in 

 moist places. There are over ten thousand known species 

 in the class, about nine-tenths of which are marine. 



Many are scavengers, feeding on any dead organic matter, 

 and thus doing a great service in helping to keep the shores, 

 small pools, and even the sea clean. Some prefer a vegetarian 

 diet and may do much damage to wood that is in the water 

 or even to crops on the land. The lobsters, crayfishes, crabs, 

 shrimps and prawns furnish man with an abundant supply of 

 most dainty food, and more important still, they furnish an 

 almost inexhaustible supply of food to many other aquatic 

 animals. 



The name Crustacea refers to the covering or crust (exo- 

 skeleton) that protects the softer parts of the body and serves 

 for the attachment of the muscles. This crust may be very 

 hard, as with the crabs, or soft and delicate, as with some of the 

 smaller forms. 



The Crayfish. The crayfish will serve as a typical example 

 of the class, but the group contains many remarkable and 

 important deviations from the type. The body is divided 

 into two well defined regions, the cephalothorax and the ab- 



