COPEPOD GENUS HEMICYCLOPS — GOODING 175 



is thus sufficient reason for considering the "other things" not to be 

 "equal" (the present wording of Article 28 of the Rules of Zoological 

 Nomenclature), and hence in the interests of stability, I have preferred 

 thyanotus to callianassae for designating this Pacific Coast species. 



It should be noted that MacGinitie's order of listing (1935) cannot 

 be used to support this choice (i.e., by dating Wilson's species from 

 his paper) since, although technically the first mention of thysa/notus 

 and callianassae in the literature, his names per se must be considered 

 nomina nuda. 



H. thysanotus seems very close to thomsoni (Canu) and to dilatatus 

 Shen and Bai. For the present, however, the following characters 

 will serve to diagnose the species: the fusion of first abdominal and 

 genital somites; the ornamentation of the antenna, particularly the 

 heavy ciliation on one of its terminal setae; the armature of the last 

 maxillar segment; the presence of setule-bearing spines on the exo- 

 podite of the first swimming leg only; the shape and ornamentation 

 of the distal segment of the fifth legs; and the modified setae on the 

 caudal rami. Specimens in the northerly part of its range are, as 

 might be expected, larger and their appendages, particularly with 

 respect to armature and ornamentation, often better developed; but 

 no meristic differences among specimens have been observed. Nor 

 could I find evidence for the existence of host-specific forms. 



Distribution: The species is known to range from San Juan Island, 

 Washington, to Monterey Bay, California, and occurs on four hosts, 

 the nudibranch Hermissenda crassicornis and the thalassinids Ca/li- 

 anassa gigas, C. califomiensis, and Upogebia pugettensis. All of these 

 hosts are common within the copepod's entire range, but there are 

 some puzzling irregularities in its geographical distribution on them. 

 From Oregon south, thysanotus seems to be abundant on Upogebia 

 and, in California (MacGinitie, 1935; MacGinitie and MacGinitie, 

 1949), on Hermissenda and C. califomiensis; yet over a period of 2)i 

 years of collecting, I have not found specimens on an} r of these three 

 in the Puget Sound region. The statement in Gooding (1958), that 

 thysanotus occurred on C. califomiensis at Nanaimo was based on a 

 misindentification. Locally, but apparently not in California, it is 

 abundant on C. gigas. 



The ecological factor common to Hermissenda and callianassids as 

 habitats is also difficult to visualize. In association with the latter 

 animals, thysanotus is generally found in the gill-chamber but may 

 also occur on the surface of the body. Although it may leave its host 

 for a short time when the latter is placed in a dish of seawater, the 

 copepod has never been found in water from the burrows. On Hermis- 

 senda, thysanotus occurs on the body surface (MacGinitie). Nothing 

 is known about its feeding habits; MacGinitie (1935) suggests that it 



53441G— 60 3 



