6go SCIENCE PROGRESS 



One of the most important and unexpected biological results 

 of the recent explorations in Antarctic seas has been the dis- 

 covery of ten-legged Pycnogons. The first of these was 

 described by Mr. T. V. Hodgson, shortly after the return of 

 the "Discovery" expedition, under the name Pentanymphon 

 antarcticnm. As a matter of fact, however, a decapodous 

 Pycnogon had been described so long ago as 1837 by Eights, 

 but his description had remained unknown to the majority of 

 naturalists, while those who knew of it either misinterpreted 

 his account or set it aside as erroneous. Shortly after describing 

 Pentanymphon, Mr. Hodgson had the good fortune to discover, 

 among the collections of the Scottish National Antarctic 

 Expedition, the long-lost Decolopoda australis of Eights, and 

 his redescription of it was published almost at the moment 

 when another naturalist was confidently asserting the im- 

 possibility of its existence. 



The possession by some Pycnogonida of five, instead of the 

 usual four pairs of legs necessitates a reconsideration of our 

 views as to their morphology and affinities, and it is hardly 

 too much to suppose that, if the ten-legged species had chanced 

 to be discovered before those with eight legs, the Arachnid 

 view of their affinities would not have received so early or 

 so general acceptance. 



At first sight Pentanymphon and Decolopoda would seem to 

 give support to the theory of Lankester and Pocock outlined 

 above. Having once, as it were, encroached upon the meso- 

 somatic region of the body in searching for homologies for the 

 Pycnogon legs, the necessity for taking two pairs instead of 

 one pair from this region presents no additional difficulty. To 

 Carpenter's view, on the other hand, the presence of a fifth 

 pair of legs offers a considerable obstacle, and destroys 

 the simplicity of the comparison between Pycnogons and 

 Arachnids. 



When we come to examine more closely the relations in 

 which the ten-legged species stand to the other Pycnogons, 

 fresh difficulties arise. It might have been expected that two 

 forms presenting so important a divergence from the normal 

 structure of the group would prove to be closely related to 

 each other. This, however, is by no means the case. Penta- 

 nymphon differs in no respect, save in possessing an additional 

 pair of legs, from the genus Nymphon, which is in many respects 



