PHYLUM ARTHROPODA 



26l 



Subclass Arachnida 



Mainly terrestrial animals, the most familiar being characters 

 the spiders and scorpions. The group also includes the ofarachnids 

 mites, of which many are aquatic, some living in the 

 sea. There is also a remarkable and aberrant group, 

 with small body and very long legs, known as sea 

 spiders (Pycnogonida), and found exclusively in the sea. 

 These differ so much from the typical arachnids that 

 they should probably form a separate subclass. The 

 most ancient arachnids appear to be those with a dis- 

 tinctly segmented abdomen, such as the scorpions and 

 harvest men. Even the spiders, which today are char- 

 acterized by the unsegmented abdomen, formerly had 



Photograph by W. P. Hay 



FIG. 74. Photograph of two sea spiders or pycnogonids. The fragment of coral 

 over which they are crawling is incrusted with bryozoans ; about natural size. 



