ART. 5. NORTH AMERICAN PARASITIC COPEPODS WILSON. 45 



Color (preserved material) a uniform yellowish brown. 



Total length, 1 mm. Width of cephalothorax, 0.45 mm.; of body, 

 0.40 mm. 



Specific characters of naupUus. — Body of medium size and ellip- 

 tical in outline, the posterior end somewhat pointed, the width about 

 two-thirds of the length. Of the appendages the first antennae are 

 the stoutest and are armed with the longest plumose setae. The 

 second antennae and mandibles are rather slender, but carry the usual 

 number of setae, four on the endopod and two on the exopod. The 

 balancers are long and slender and are slightly enlarged at the base ; 

 the terminal portion is bent like the bowl of a spoon. They are car- 

 ried well forward almost at right angles to the long axis of the body. 

 In the egg strings these nauplii appear cinnamon brown by reflected 

 light, but when examined by transmitted light the pigment changes 

 to a dark olive green. This pigment occupies the whole center of the 

 bodj' and is arranged in the shape of a shield, the anterior end nar- 

 rowed and convexly truncated, with small prominent corners, the 

 posterior end bluntly pointed. Outside of this pigment mass is a 

 strip of transparent mesenchyme, whitish or grayish in color, very 

 narrow posteriorly, wider and denser anteriorly. Outside of the 

 mesenchyme is a narrow clear margin, formed by the fusion of the 

 dorsal and ventral portions of the cuticle. 



The muscles which move the appendages are quite distinctly shown, 

 especially through the mesenchyme. 



Total length, 0.33 mm. Greatest width, 0.25 mm. 



Vascular systeni. — Looked at from the dorsal surface we find a 

 single central, longitudinal trunk, considerably enlarged in the ceph- 

 alothorax and free thorax, and tapering gradually posteriorly. In 

 the center of the cephalothorax is a pocket or receptable, out of which 

 leads posteriorly the enlarged longitudinal trunk, and anteriorly a 

 short branch to each anterior corner of the head ; there is also a short 

 branch from the center of either side of the pocket. The longitudinal 

 trunk gives off two branches near the posterior margin of the cepha- 

 lothorax on either side, a single branch at the center of the free 

 thorax, and a large branch in the genital segment leading to each 

 fourth leg. In the carapace and the two portions of the dorsal plate, 

 and in the laminae of the fourth legs these branches anastomose into 

 a network of fine tubes which covers the entire surface. Within 

 these tubes the vascular liquid is freely exposed to oxygenation. On 

 the ventral surface we find two longitudinal trunks which give off 

 branches to the various appendages. 



At the anterior end are two branches which ramify towards the side 

 lappets of the cephalothorax, in which there is often a profuse net- 

 work of capillaries. Then follow branches to the second antennae, the 

 second maxillae, the maxillipeds, and the swinging legs. The capil- 

 313G— 22— Proc.N.M.Yol.60 11 



