ORGANIC STRUCTURE : MORPHOLOGICAL 39 



tacea, Insecta, Spiders, Mites, Myriapoda, etc. (taking account 

 of the fossil forms). They are the most widely distributed of 

 animals. 



The Arthropod has a jointed body with jointed limbs, or 

 appendages. The bodily joints divide the body into segments. 

 The body is typically enclosed in a calcareous shell, or carapace. 

 Exceptionally the limbs may be absent and there may be no hard 

 carapace. Usually some, or (exceptionally) all of the segments 

 coalesce. Typically each segment carries a pair of appendages 



AUjThentarif 

 cojuxZ 



Nerve 

 cord - 



EocosheletoTL 



AppendjoLges 



AtLmentarif 

 canal 



PcurapodijjL Mrvecord CoeLorn 



2a 



Pectoral 

 limh (fin) 



o Alimentary 

 ^ cojial^--^ 



Endo- 

 skeletoTi 



5a 



Pelvic liwh (fin,) 

 Fig. 9. 



CoeioTTL 



I, A crustacean ; la, transverse section of the body of the same ; 2, a worm ; za, trans- 

 verse section of the body of the same ; 3, a vertebrate (fish) ; 3a, transverse section of the 

 body of the same. 



and the forms of the latter are usually modified for various 

 purposes (walking, swimming, prehension, aggression, sensation, 

 etc.). Symmetry is triple. In spite of the most extraordinary 

 modifications the above essential structure is either obvious in 

 the adult arthropod or can be seen in the embryogeny. 



11/. The Chordate Type. This includes the vertebrates, 

 the tunicates and some other infrequently occurring forms. The 

 symmetry is triple (with a curious radial arrangement in the 

 Colonial Tunicates). The chordates are characterized by the 

 presence of a notochord, which becomes the vertebral column. 

 There is an internal limy endo-skeleton in most chordates in 



