PHYLOGENY OF ARIETELLID COPEPODS 



and 2 aesthetascs. Antenna: second endopod segment (Fig. 

 16G) with 2 short inner setae medially and 5 setae and 1 

 vestigial seta terminally. Mandibular palp (Fig. 18D): first 

 exopod segment with reduced, short seta. 



Male. Cephalosome separate from first pedigerous somite. 

 Left antennule exhibiting same fusion pattern and armature 

 elements as A. plumifer except for first segment: I-IV-7 + 7 

 aesthetascs (1-3 + aesthetasc, II — 1 + 2 aesthetascs, III — 1 + 2 

 aesthetascs, IV-2 + 2 aesthetascs). Antenna: second endo- 

 pod segment (Fig. 16F) bearing 1 long and 1 short seta 

 medially. Mandibular palp (Fig. 18E): first exopod segment 

 with well-developed seta. Maxillule: endopod almost fused 

 with basis, represented by small knob. Maxilliped: fourth and 

 fifth endopod segments each having vestigial innermost seta, 

 as in A. plumifer. Leg 5: left endopod (Fig. 180) indistinctly 

 2-segmented, with suture visible on posterior surface; com- 

 pound distal exopod segment of right leg with minute termi- 

 nal element. 



Remarks. A. aculeatus exhibits sexual dimorphism in the 

 antenna and mandibular palp, as does A. plumifer. 



Arietellus setosus Giesbrecht, 1892 (Figs 16J,18I,M) 



Material examined, d". 



Body length. 4.28 mm. 



Description. Male. Cephalosome separate from first pedi- 

 gerous somite. Left antennule with same fusion pattern and 

 armature as A. plumifer. Antenna: exopod indistinctly 

 7-segmented; setal formula 0,1,1,1,1,0,3. Mandible: first exo- 

 pod segment with normally developed seta. Maxillulary 

 endopod (Fig. 16J) represented by unarmed, small knob. 

 Maxilla and maxilliped (Fig. 181) as in A. plumifer. Leg 5: left 

 endopod (Fig. 18M) indistinctly 2-segmented as in A. plumi- 

 fer, first segment produced ventrally to rounded tip, second 

 segment rising from inner side of first segment; terminal spine 

 on third exopod segment of left leg almost completely fused 

 to segment, subterminal spine incompletely coalesced with 

 segment; distal compound exopod segment of right leg 

 unarmed. 



Arietellus pavoninus Sars, 1905 (Figs 16B,H,17B,18J) 



Material examined. 9- 



Body length. 5.00 mm. 



Description. Female. Cephalosome separate from first 

 pedigerous somite. Genital double-somite (Fig. 16B) similar 

 to that of A. plumifer, but readily distinguishable since 

 seminal receptacle relatively much larger than in A. plumifer, 

 over half length of genital double-somite. 



Antennule with same fusion pattern and armature as A. 

 plumifer except for absence of aesthetasc on segment IX (this 

 aesthetasc may have been detached). Mouthparts similar to 

 those of female A. plumifer except for maxillulary endopod. 

 Maxillule (Fig. 16H): endopod distinctly 1-segmented, bul- 

 bous with 1 bipinnate seta. Maxilliped (Fig. 18J): fourth and 

 fifth endopod segments each with reduced innermost seta, 

 sixth segment with reduced setae a and b. Leg 5 (Fig. 17B): 

 coxae incompletely fused with intercoxal sclerite, in particu- 

 lar, more fused in right leg; endopod represented by 2 

 plumose setae not so produced as in A. plumifer; exopods 



131 



1-segmented, separate from basis, carrying 1 unipinnate spine 

 terminally. 



Arietellus simplex Sars, 1905 (Figs 16E,I,18N) 



Material examined, d". 



Body length. 6.10 mm. 



Description. Male. Cephalosome separate from first pedi- 

 gerous somite. Left antennule with same fusion pattern and 

 armature as A. plumifer. 



Antenna: exopod (Fig. 16E) indistinctly 8-segmented; setal 

 formula 0,1,1,1,1,0,0,3. Mandible: first exopod segment with 

 normally developed seta. Maxillule: endopod represented by 

 low knob, almost fused with basis (Fig. 161). Maxilliped as in 

 A. plumifer. Leg 5: left endopod (Fig. 18N) indistinctly 

 2-segmented, suture visible on both surfaces; terminal and 

 subterminal spines on third exopod segment of left leg 

 incompletely fused to segment, terminal spine with 4 minute 

 spinules terminally; terminal spine of distal compound exo- 

 pod segment of right leg unarmed. 



Arietellus sp. (Figs 16C,D,17D,18C,G,K) 



Material examined. $. 



Body length. 5.15 mm. 



Description. Female. Cephalosome separate from first 

 pedigerous somite. Posterolateral angles of prosome asym- 

 metrically produced into sharp lateral processes as in A. 

 giesbrechti (see Sars, 1924, 1925), left process slightly longer 

 and more produced than right. Genital double-somite (Fig. 

 16C) similar to that of A. mohri in having pair of laterally 

 expanded copulatory ducts, but differing in presence of better 

 developed muscles to genital operculum. 



Left antennule with same segmentation and armature as A. 

 plumifer. Antennary endopod: first segment unarmed, sec- 

 ond (Fig. 16D) with 1 long and 1 short seta medially, and 5 

 setae and 1 vestigial element terminally. Mandibular palp 

 (Fig. 18C) with relatively long seta on first exopod segment. 

 Maxillulary endopod completely fused with basis. Maxilla: 

 basal spine (Fig. 18G) with 2 rows of spinules along ventral 

 margin. Maxilliped (Fig. 18K): fourth and fifth endopod 

 segments lacking innermost seta; sixth endopod segments 

 with setae a and b reduced. 



Leg 5 (Fig. 17D) similar to that of A mohri with intercoxal 

 sclerite, coxa, basis and both rami almost completely fused, 

 but distinguishable by: seta on both endopods represented by 

 low knob much better developed than in A. mohri; unarmed, 

 lobate exopods more developed than in A. mohri; left basal 

 seta longer than in A. mohri. 



Remarks. Arietellus sp., an as yet undescribed species, is 

 most closely related to A. mohri in having synapomorphic 

 characters such as no innermost seta on the fourth and fifth 

 endopodal segments of maxilliped and the reduced leg 5. 



Genus Rhapidophorus Edwards, 1891 



Type species. Rhapidophorus wilsoni Edwards, 1891 

 (monotypic). 



Remarks. Fosshagen (1968) first pointed out the affinity of 

 this genus with Paramisophria. Campaner (1977) later 

 assigned the genus to the family Arietellidae. The genus. 



