1.0 mm 



FIGURE 2. — Mouthparts from right side of megalopa ofChionoecetes bairdi (a) mandible; (b) maxillule; (c) maxilla; (d) first maxilliped; 

 (e) second maxilliped; (f) third maxilliped; (g) ventral view of sternum and pleopod attachment; (h) ventral view of telson and uropods; 

 (i) lateral view of abdomen. 



closely resemble those of adult. Coxopodite and 

 basipodite spines, one each, located ventrally on 

 chelipeds and ambulatory legs except for fourth 

 leg. First ambulatory leg spines especially long. 

 Cheliped and third ambulatory leg spines minute. 

 Dactylopodites of ambulatory legs one, two, and 

 three have conspicuous spine projecting from tip. 



ABDOMEN AND TELSON (Figure 2h, i)— 

 Abdomen six segmented. Sixth segment and tel- 

 son small. No spines present. Segments two 

 through five have long setae on dorsal surface. 



PLEOPODS (Figure 2i)— Pleopods present on 

 abdominal segments two through five. A single- 

 segmented endopodite (not shown in figure) arises 

 from proximal segments of each pleopod. Endo- 

 podites have four hooked setae on distal end of first 

 three pairs of pleopods and three hooked setae on 

 distal end of last pair of pleopods. Exopodites of 

 pleopods two and three have variable numbers of 

 plumose setae, 15 through 18. Exopodites of 

 pleopods four and five have 17 and 15 plumose 

 setae, respectively. 



UROPODS (Figure 2h)— Uropods two seg- 

 mented and have seven plumose hairs arising 

 from each distal segment. 



How to Distinguish Megalopa of 



Chionoecetes bairdi, C. opilio, 



and C. opilio elongatiis 



Megalopa of C. bairdi are similar to megalopa 

 of C. opilio and C. opilio elongatus in major 

 carapace spination and size. The characteristics 

 which separate these species can be determined 

 without dissection. The four most useful charac- 

 teristics are: 1) C. bairdi has a minute lateral 

 spine in the region of the pterygostomial- 

 branchial ridge while the others do not (see 

 Kurata 1963b; Motoh 1973); 2) C. bairdi has a 

 more pronounced ridge along the posterior margin 

 of the carapace than C. opilio and C. opilio elon- 

 gatus (Kurata 1963b; Motoh 1973); 3) the rostral 

 spine of C. bairdi is three times the length of the 

 preorbital spines, whereas the rostral spine on C. 



462 



