26 DISCOVERY REPORTS 



An examination of the syntypes showed that in specimens measuring under 5 mm. 

 in trunk length the tarsus is either shorter than, or equal to, the propodus. Thus it 

 would appear that the tarsus is short in young specimens and, as growth proceeds, it 

 elongates more rapidly than does the propodus. Also, in immature specimens the femur 

 is much shorter than, and the third walking leg (claw excluded) is 2-5-4 times as long 

 as, the trunk. In adults the femur is subequal to or longer than, and the leg is 4-6-5-8 

 times as long as, the trunk (see measurements). In these respects the specimen from 

 St. 170 differs markedly from small specimens of P. antarcticum. This species of five- 

 legged Pycnogonida might prove of interest to the student of heterogonic growth in 

 Arthropoda. 



Genus Nymphon, Fabricius, 1794 



A revision of the large genus Nymphon, comprising some ninety species, although 

 a long and tedious business, is much needed. In addition to the fact that many species 

 are very imperfectly described, the characters that serve to distinguish one species from 

 another have never been comprehensively studied. It has not been possible to devote 

 time to a complete revision of the genus at present ; the study was therefore limited to 

 Antarctic forms. 



While much work has been done on material from Antarctic and sub-Antarctic 

 waters during the last thirty years,i the relationships of the species are very imperfectly 

 known and the key to their determination given by Loman (1923, p. 14 — a modification 

 and extension of that of Bouvier, 1913, p. 72) I found to be far from satisfactory. In 

 order to identify the Discovery material with any degree of accuracy a revision- of all 

 the Antarctic species had to be undertaken. 



In several instances the type specimen is immature (TV. frigidum, Hodgson ; A^. longi- 

 colliim, Hoek; A^. lanare, Hodgson and A'^. trideiitatiim, Hodgson), while not a few species 

 are known only from the female (see Table III). The species in which the adult male is 

 known fall into two main groups, each with several atypical or aberrant forms as listed 

 below (see also Table HI). The larger=^ (group I) includes most of the species referred 

 to the genus "Nymphon"^ by those authors who recognize two genera (e.g. Hodgson, 

 Bouvier, Loman); the smaller (group II) includes those species referred to the genus 

 " Choeto?ivmphon'' as well as several forms hitherto regarded as belonging to "Nym- 

 phon", viz. A^. capense, Hodgson, A^. cotnpoctum, Hoek, and A^. articidare, Hodgson. 



The fact that the Antarctic species fall naturally into two main groups would seem to 

 be strong evidence in support of the validity of the genus Chaetonymphon, Sars. But 

 a careful study of all the characters, many of which are represented in tabular form 



1 E.g. Hodgson, 1907, 1908; Bouvier, 1906, 1913; Caiman, 1915; Loman, 1923. 



- The types of all the species included in Table III have been re-examined, with the exception of 

 N. subtile, Loman; the Discovery specimens, however, are from the same locality as, and agree with Loman's 

 description of, the type. (See also Gordon, 1932.) 



^ Seven of the species known only from female or immature specimens (Table III B) probably belong 

 to group I ; A'^. villosum, Hodgson belongs to group II and is allied to .V. biwticulatum and A^. brevicaudatum. 



* When the word Nymphon is enclosed in inverted commas in the text it means Nymphon scnsu strkto of 

 certain authors, i.e. group I and the species listed above now placed in group II. 



