m PHYLUM ARTHROPODA 670 



The position of the Pentastomida is a matter of uncertainty. 

 In the absence of organs of respiration and excretion, the only 

 feature in the adult which distinctly points to arthropod affinities 

 is the striated character of the muscular tissue. The presence of 

 two pairs of legs in the larva, however, is sufficient to confirm the 

 view that they are aberrant and probably degenerate Arthro- 

 pods, while leaving it uncertain in what class they find their near- 

 est allies. The Tardigrada are still more aberrant in some respects. 

 They differ from Arthropods in general in the absence of external 

 segmentation in the adult state, in the simple unjointed character 

 of the appendages, in the absence of striation in the muscular 

 fibres, and in the absence of organs of respiration and circulation. 

 It is impossible to place them in any of the great classes, and they 

 are perhaps best looked upon as a special offset of the Arthropod 

 tree given off near the base. 



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