RELATIONS OF INSECTS TO OTHER ARTHROPODA 3 



composed of segments (somites or arthromeres) bearing jointed ap- 

 pendages. They differ from the worms in having segmented ap- 

 pendages, i.e. antennae, jaws, and legs, instead of the soft unjointed 

 outgrowths of the annelid worms. Moreover, their bodies are com- 

 posed of a more or less definite number of segments or rings, grouped 

 either into a head-thorax (cephalothorax) and hind-body, as in Crus- 

 tacea, or into a head differentiated from the rest of the body (trunk), 

 the latter not being divided into a distinct thorax and abdomen, as in 

 Myriopoda; or into three usually quite distinct regions the head, 

 thorax, and hind-body or abdomen, as in insects. In certain aberrant, 

 modified forms, as the Tardigrada, or the Pantopoda, and the mites, 

 the body is not differentiated into such definite regions. 



In their internal organs arthropods agree in their general relations 

 with the higher worms, hence most zoologists agree that they have 

 directly originated from the annelid worms. 



The position and general shape of the digestive canal, of the 

 nervous and circulatory systems, are the same in Arthropoda as in 

 annelid (oligochete) worms, so much so that it is generally thought 

 that the Arthropoda are the direct descendants of the worms. It is 

 becoming evident, however, that there was no common ancestor of 

 the Arthropoda as a whole, and that the group is a polyphyletic one. 

 Hence, though a convenient group, it is a somewhat artificial one, 

 and may eventually be dismembered into at least three or four phyla 

 or branches. 



The following diagram may serve to show in a tentative way the 

 relations of the classes of Arthropoda to each other, and also may be 

 regarded as a provisional genealogical tree of the branch. 



9. Insecta. 

 4. Arachnida. 



3. Merostomata. 

 1. Crustacea. 



2. Trilobita. 



I 7. Chilopoda. 



6. Diplopoda. 

 8. Symphyla. 



6a. Pauropoda. 



4a. Pantopoda. 



b. Tardicjrada. 



5. Peripatus. 



Different Annelida. 



Trochusiihwra. 



