ANIMALS WITH JOINTED LEGS— THE ARTHROPODS 



183 



Characteristics of the Phylum 



The phylum name, Arthropoda, comes from Greek words meaning 

 jointed feet and this is one of the most distinguishing features of the 

 group. The feet, or legs, consist of a series of articulating joints which 

 may be highly modified for specialized functions, such as swimming, 

 respiration, obtaining food, etc. Members of this phylum have seg- 

 mented bodies, such as are found in the annelids, but the segments are 

 not all similar to one another like the annelids, but may be fused or modi- 

 fied to meet specialized needs. They have an exoskeleton of chitin which 



Fig. 13.2. Legs of arthropoda, showing the jointed condition characteristic of this 

 phylum. From left to right they are the legs of a praying mantis, roach, spider, and 



crayfish. 



is a very efficient skeleton. The term skeleton is commonly associated 

 with an assortment of bones including a skull, ribs, vertebrae, and arm 

 and leg bones ; yet the outer covering of a grasshopper is just as much a 

 skeleton even though it is found on the outside of the body. This is 

 called the exoskeleton, which has its muscles inside, as contrasted to the 

 endoskeleton, which has its attached muscles on the outside. Both serve 

 their purpose very well, but there are certain advantages and disadvan- 

 tages of each which might be considered. 



The exoskeleton has a great advantage in protection. With the soft 

 body parts internal to the skeleton they are much better protected than 



