ABSTRACT
The following discussion defmes both the phylum Arthropoda and the superclass Hexapoda. Details of the external features will be given in the next chapter.
All arthropods share the feature of having an exoskeleton composed of chitin and sclerotin, a complex proteinlike chemical, and sometimes other chemical materials such as calcium (lacking in all insects). The most distinctive feature, however, is their segmented appendages; that is, appendages divided into parts separated by joints. These parts are moved either by muscles that are contained within each one (making it a true segment) or by tendonlike connections attached to internal muscles at the base of the series of segmendike parts. These connections are called merons, and this term is added to the name of those parts that are not true segments, merely segmendike.
