29 



Artnropoas — Crustaceans 



There are more kinds of arthropods than of all other animals together (Fig. 

 29.1). They are a collection of multitudes: crustaceans, hosts of little ones as 

 well as large lobsters and crabs; myriapods, the centipedes and millipedes; 

 spiders and their allies, ticks and mites; and insects by millions. Their variety 

 seems infinite but their basic pattern is the same, the tube within a tube plan 

 of body and the segmentation inherited from annelid worms. Two leading 



Fig. 29.1. The relative abundance of 

 species of arthropods is estimated to be 80 

 per cent of all kinds of animals. (Courtesy, 

 Frost: General Entomology. New York, 

 McGraw-Hill Book Co., 1942.) 



characteristics have developed upon this ground plan, an important and com- 

 plex head, and jointed appendages the unique character of which has given 

 the name Arthropoda {arthros, a joint -f pous, a foot) to the phylum (Fig. 

 29.2). 



Arthropods have complex and important relationships with plants and 

 with other animals. Among them are the social organization of ants and bees, 

 the most elaborate outside of human society, the effects of insects upon agri- 

 culture throughout the world, constructively by cross pollinating plants, de- 

 structively by feeding on plants and by carrying diseases from one to another 

 plant, animal or man. Crustaceans provide the chief food for many fishes; 



572 



