THE PROBLEM OF THE PYCNOGONS 



By W. T. CALMAN, D.Sc. 



The small group of marine animals known to zoologists as 

 Pycnogonida, Pantopoda, or Podosomata, and by the more 

 popular but somewhat misleading names of " Sea Spiders " or 

 " Nobody Crabs," have always been a stumbling-block to those 

 who have attempted to draw up a natural system of classification 

 for the Arthropoda. They have been ranked sometimes with 

 the Arachnida and sometimes with the Crustacea, they have 

 been placed by themselves as a separate division of the Arthro- 

 poda, 1 and one investigator has even attempted to demonstrate 

 their affinities with the Myriopoda. The recent discovery, in 

 Antarctic Seas, of two surprisingly aberrant members of the 

 group, instead of throwing light on their systematic relations, 

 has only added to the difficulty of interpreting them. 



A typical Pycnogon, such as Nymphon, has a slender body 

 composed of a cephalic segment or " head," followed by three 

 free body-segments (somites) and a small terminal lobe known 

 as the abdomen. Four pairs of disproportionately long legs are 

 attached, the first pair to the cephalic segment and the others to 

 the three following body-segments. In addition, the cephalic 

 segment bears three pairs of appendages, the first being chelate 

 (pincer-like) and overhanging a long tubular proboscis which 

 bears the opening of the mouth ; the second pair are sensory 

 " palps" placed at the sides of the proboscis ; and the third pair, 

 immediately behind these, have, in the male sex, the function 

 of carrying the eggs deposited on them by the female, and are 

 therefore known as "ovigerous legs" or " ovigers." 



It is at once apparent that there can be no close comparison 

 between the plan of structure here outlined and that of any 



1 For the benefit of those unfamiliar with zoological classification it may be 

 explained that the comprehensive group Arthropoda includes the classes Crustacea 

 (crabs, lobsters, shrimps, etc.), Arachnida (scorpions, spiders, king-crabs, etc.), 

 Diplopoda (millipedes), Chilopoda (centipedes), Hexapoda (insects), and Onycho- 

 phora (Perifiatus). The Diplopoda and Chilopoda are sometimes regarded as 

 forming a single class, Myriopoda. 



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