662 ZOOLOGY SECT, xi 



of the various groups of Arthropods by some such scheme as that 

 expressed in the diagram (Fig. 571). 



Here an intermediate link between Annelida and the existing 

 Arthropoda is supposed to have been constituted by hypothetical 

 primitive forms from which Peripatus, the Insecta, and the 

 Myriapoda are supposed to have been evolved in the one direction, 

 and the Crustacea, Eurypterida, Xiphosura, and air-breathing 

 Arachnida in the other. 



On account mainly of general resemblances to the Spiders, the 

 Pycnogonida have frequently been grouped with the Arachnida, 

 and attempts have been made to homologise their appendages 

 with those of the Spiders and Scorpions. There is at least one pair 

 more in the Pycnogonida ; and either the last pair would have to 

 be set down as corresponding to the vestigial first abdominal pair 

 of the ordinary Arachnida, or the ovigerous legs would have to be 

 reckoned, not as independent appendages, but as parts of the 

 second pair, a view for which there is some ground. A close 

 relationship with the Arachnida, however, cannot be traced, and 

 their affinities are perhaps best expressed, as in the diagram, by 

 connecting them with the Arachnid branch of the Arthropod tree 

 at a point below that at which the air-breathing forms had become 

 developed from forms allied to the Xiphosura. 



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 Arthropods, 

 while leaving it uncertain in what class they find their nearest 

 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. 



