116 PROCEEDIXGS OF THE NATIONAL MUSEUM vol.86 



Commensal, in ambulacral grooves of echinoderms or branchial 

 chambers of mollusks. 



Type species: Arctonoe vittata (Grnbe), new combination. 



Discussion. — The genus Arctonoe includes a small group of anne- 

 lids that differ, by degrees, in the reduction or loss of certain parts, 

 such as the number of notopodial setae, presence or absence of ventral 

 cirri, number of segments and of scales, size of eyes. The habit of com- 

 mensalism is possibly largely responsible for certain of these reduc- 

 tions or deletions. All known species are similar in that they have the 

 peculiar, heavy, falcate neuropodial setae adapted for attaching to 

 their hosts, and in other respects enumerated above. 



The first species of this group, Polynoe vittata^ was described from 

 Alaska by Grube (1855, p. 82). It has subsequently been delegated to 

 the genera Lepidonotus (Baird, 1863, p. 107), Hdlosydna (Baird, 1865, 

 p. 190), Acholoe (Marenzeller, 1902,p. 576),andjEraZ6>s?/<^no2V;?es (Seid- 

 ler, 1921, p. 134). It has been described as Lepidonotus lordi Baird 

 (1863, p. 107), as Ilalosydna succiniseta Hamilton (1915, p. 234 [new 

 syn.]), and more recently as Arctonoe lia Chamberlin (1920, p. 6B 

 [new syn.]). Since Chamberlin's name Arctonoe precedes Halosyd- 

 noides Seidler (1924), the former is used. 



There are now four known species that may be assigned to the genus 

 Arctonoe. They are : PoJynoe vittata Grube (1855. p. 82) . Lepidonotus 

 fragilis Baird (1863, p. 108), Polynoe pidchra Johnson (1897, p. 177), 

 and narmothoe tuherculata Treadwell (1906, p. 1154) (see below). 

 The first three of these have not been reported outside of the north 

 Pacific, and A. pidchra and A. fragilis are knov.n onl}' in the north- 

 east Pacific. A. tuherculata is known only from the Hawaiian Islands 

 (Treadwell, 1906). 



An interesting correlation of host species may be observed. All are 

 more or less commensal with asteroids. A. vittata is more commonly 

 with fissurellids, though sometimes also on chitons and even in tube of 

 TheJepus (Berkelej^, 1935, p. 212) . A. pulchra is more frequently with 

 holothuroids. 



KEY TO THESE SPECIES OF ARCTONOE 



1. Dorsum of each segment with a consi)icuous median papilla ; noto- 



podial setae absent ; neuropodial setae with vestiges of pectinae 



(fig. 37, e) tuberculata 



Dorsum without such papillae; notopodial setae present on at 

 least a few anterior segments though often reduced in size and 

 number; neuropodial setae without pectinae 2 



2. External margins of elytra greatly ruffled or folded frag'ilis 



External margins of elytra smooth or only slightly ruffled 3 



3. Some superior neurosetae with bifid tip; dorsum usually with a 



darii pigment band across segments 7-S vittata 



Superior neurosetae resembling inferior ones ; dorsum without 

 transverse band of pigment pulchra 



