190 



HIGH SCHOOL ZOOLOGY. 



CHAPTER VII. 



The Arthropoda. 



1. It was stated in Chapter I., that the term Invertebrata in- 

 cludes several distinct sub-kingdoms, resembling each other 

 in that they do not possess the arrangement of the nervous 

 system and skeleton typical for the Vertebrates. It must not 

 be understood that all are equally unlike Vertebrates, some 

 worms for example, seem to foreshadow in their structure the 

 vertebrate organization, but the most highly organized Inverte- 

 brates — the Arthropoda — ^di verge very widely from the type of 

 structure which we have studied heretofore, although their 

 organs are built up of histological elements, similar, in many 

 res|)ects, to those of the Vertebrates. The nature and extent of 

 the divergence referred to may be studied in the crayfish, an 

 Arthropod which forms a suitable introduction to the sub- 

 kingdom to which it belongs, both on account of its size and on 

 account of its position in the group, 



2. The Arthropoda are bilaterally segmented animals like the 

 Vertebrates, but, unlike them, their segmentation is visible on 

 the surface of the body, especially on ae count of the fact (from 

 which the group derives its name) that each segment may carry 

 one pair of jointed appendages. Throughout the sub-kingdom 

 the rule holds good which obtains also in the Vertebrates, that, 

 in the more primitive families, the segments are not only more 

 numerous but less constant in number, and show less tendency 

 to be grouped into dissimilar regions. The nervous system par- 

 takes in the segmentation of the body, but is situated on the 

 ventral as^iect, wliile the centre of the blood-vascular systejn is 

 dorsal, so that there is a complete reversal of the neural and 



