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MARINE BIOLOGICAL LABORATORY. 



abdominal appendage, so that the difficulties arising from 

 the existence of similar openings in the Pycnogonid, on 

 most of the appendages, is partially at least overcome. 

 I have also examined the structure of the adult eyes of 

 the Pycnogonid, which seem to furnish, perhaps, an 

 explanation to the early inversion of the eyes of the 

 Scorpion and other Arachnids. 



Let us turn finally to an examination of the ap- 

 pendages of the Pycnogonids, where we meet with a 

 stumbling-block in the way of our acceptance of the 

 relationship of the sea-spider and Arachnid. 



If we arrange in two series the appendages of the 

 two groups, beginning with the first pair in each, we 

 get the following table : — 



In the Pycnogonids there are seen to be seven pairs 

 of appendages, while the spider, etc., have only six pairs 

 in all. There are four of these pairs used by each as 

 walking-legs, and in the Pycnogonids three pairs lie in 

 front of these, and two such pairs in the spiders. 



We may get out of this difficulty of numbers in either 

 of two ways. In the first place, we may assume that 

 the four pairs of walking-legs are homologous in the 

 two groups, and at first sight this seems very plausible. 



