PART III. 



DIVISION OR TYPE II. : ARTICULATED ANIMALS 

 (ARTHROPODA). 



Laterally symmetrical animals provided with an external chitinous 

 skeleton. Body composed of a number of annular segments usually 

 coalescing in groups (head, thorax, abdomen). Each segment may be 

 provided with a pair of articulated appendages (limbs). The main 

 portion of the nervous system placed on the ventral side of the body, 

 the heart situated on the dorsal side. 



The laterally symmetrical type of structure with which we became 

 acquainted in the case of the Vertebrata (see Part I., p. 2) is repeated in 

 insects, spiders, myriapods and crustaceans, which together form the 



Diagrammatic Representation of the Structure of an Arthropod. 



Sk., Chitinous skeleton ; G., limbs ; N., nervous system ; uS., oS., inferior and superior 

 oesophageal ganglion; S., gullet; D., intestine; H., heart; A., eye; F., antenna; 

 M. , mouth parts. 



second large division or type of the animal kingdom. The animals com- 

 prised in this division are, however, constructed on a widely different 

 plan, and therefore represent a quite distinct animal type. 



1. Skeleton. — If we open one of these animals, we never find an 

 internal bony or cartilaginous skeleton, such as in mammals gives sup- 

 port and firmness to the body and attachment to the muscles concerned 

 with locomotion (see Part I., p. 4). We are in fact in this, as in all the 

 remaining animal groups, dealing with boneless or invertebrate animals. 



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