NO. 1504. AMERICAN PARASITIC COPEPODS— WILSON. 687 



that precocious development which characterizes the more degenerate 

 families of these parasites. 



This is important in its bearing- upon degeneration, since it is the 

 very first evidence to be obtained from the development stages. And 

 its value is enhanced from the fact that it occurs in a species whose 

 adult form shows no appreciable diminution of bodily functions or 

 morphology. The adults of both sexes in this genus swim as freely 

 and as actively as any Caligus. Their fourth swinmiing legs, to be 

 sure, have been reduced to mere stumps, and the fourth segment, 

 which carries them, is covered by a pair of small dorsal plates. 

 But, even in this condition, they are about as serviceable as the cor- 

 responding members in Caligus; that is, thev are of no real service in 

 either case so far as can be determined. 



The e3'es are situated well back toward the tenter of tne carapace 

 and are relatively very large. The pigment is not as extensive as in 

 the Caligina3, the lens being surrounded by a large, clear area. Over 

 the dorsal surface also, in place of the broad lateral pigment lines and 

 the large area in front of the eyes, we find only isolated pigment spots 

 and very few of them. There is a single small spot in the frontal 

 plate on either side at the base of the first antenna and a narrow line 

 across the posterior end of the carapace near the margin. 



There is a similar narrow line across the posterior margin of the 

 first free segment, a pair of large spots in the groove between the 

 second free segment and the abdomen, and another pair of spots at 

 the posterior end of the abdomen over the bases of the anal laminae. 

 This metanauplius, therefore, has very little pigment, while the same 

 stage of development in the Caligiuis was highly pigmented. 



The carapace is followed b}' two free thorax segments and the abdo- 

 men; only the first of the free segments bears swimming legs. The 

 abdomen at this stage is reall}^ a fusion of the fourth thoracic, the 

 genital and abdominal segments, the two former being not as yet difl'er- 

 entiated. It is as wide as the last thorax segment and terminates in 

 two rather short anal laminw, each armed with five plumose setw. 



The first antennte are two-jointed, the terminal joint l)earing remark- 

 ably long and branched sette, which are not feathered as in the Cali- 

 ginte. These setaj are remarkable in several particulars among those 

 of all the parasitic copepods thus fai examined. Thej^ are longer 

 than even the plumose rowing setie on the second antenme and man- 

 dibular palps of the Ai'gulus larva. The^' extend outward in ever}^ 

 direction like ordinary antennal seta3, but instead of being plumose 

 they are dichotomousl}^ branched toward the tips and thus terminate 

 in a flattened web or mat very similar to that formed by certain algje. 

 Evidently they have retained much of their old locomotor function 

 which the}^ possessed in the nauplius stage. 



