CHAPTER 16 



Phylum Arthropoda 



Arthropods are segmented animals whose epidermis secretes an exo- 

 skeleton of stout rings corresponding with the segments; the rings are 

 connected by flexible membranes that act as joints. Many of the seg- 

 ments bear paired lateral appendages, each of which has a similar 

 chitinous skeleton of jointed rings. The phylum takes its name from 

 these jointed appendages (Gr. arthros joint + podos foot). The exo- 

 skeleton is a chemical complex which includes chitin, a nitrogenous 

 polysaccharide made of sugar, ammonia and acetic acid. The body in- 

 cludes a head, thorax and abdomen, each composed of several seg- 

 ments which may be fused in various ways. The body musculature is 

 made up of numerous small muscles extending across joints to form 

 an intricate mechanism capable of precise complex movements. 



The evolutionary potentialities of such a structural system would 

 appear to be tremendous. The exoskeleton not only forms a protective 

 cover that has been successful in all of the habitats of the world, but 

 its division into numerous parts makes possible many different mor- 

 phologic adaptations to particular habitats. For example, the mouth 

 parts of an insect may be modified for biting, chewing, scraping or 

 sucking. The specialization of the skeletal parts of many arthropods 

 has adapted them beautifully for some particular habitat; they are so 

 precisely adapted, in fact, that they are severely limited in their ecologic 

 distribution. This may explain the enormous number of species of 

 arthropods, for many species can coexist in the same geographic region 

 if each has different ecologic requirements. At the present time the 

 known species of all other phyla add up to about 130,000, while those 

 of the arthropods alone add up to 870,000! The majority (800,000) of 

 these are insects, most of which are terrestrial. 



132. Classification of the Phylum 



Arthropods can be divided into four subphyla according to the struc- 

 tures of the appendages of the first six segments. In all arthropods the 

 first segment, believed to correspond with the annelid prostomium, ap- 

 pears in the embryo but is never distinct in the adult. It never has ap- 

 pendages. In most arthropods the mouth opens ventrally between the 

 third and fourth segments. 



The four subphyla (Fig. 16.1) are the Trilobito, Arachnomorphc, 



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