ART. 17. AMERICAN PARASITIC COPEPODS WILSON. 3 



Young male. — The dorsal view of a male about 4 mm. long is 

 shown in Figure 6 and attention is called to many differences between 

 this form and the adult. The carapace is fully a fourth longer 

 than wide, and it only reaches the anterior margin of the third 

 legs instead of overlapping the abdomen: the posterior sinus is 

 short and fully as wide as long. The arrangement of the chitin 

 ribs on the dorsal surface of the head is more distinct than in 

 mature specimens, and is different from that shown in any of the 

 published figures. The chitin ribs are strongly curved between the 

 lateral eyes, and each is forked just behind the median eye, posterior 

 to which the two are nearly parallel. The inner branch of the fork 

 curves backward and inward and meets its fellow from the other 

 side on the midline, forming a reentrant notch. The outer branch 

 curves backward and outward and nearly or quite meets the lateral 

 groove. 



Behind the posterior end of this lateral groove, and extending 

 forward a short distance on either side, is a narrow fold of skin. 

 From either end of this skin fold a groove extends backwards and 

 inwards to the corresponding corner of the base of the posterior 

 sinus. 



The abdomen is nearly one-third of the entire length, and is also 

 one-half longer than wide. It is widest at the center instead of at 

 the anterior end and shows but the faintest indication of the finger- 

 like processes that later appear at the anterior corners. The testes 

 are half in front and half behind the longitudinal center, whereas 

 later they are wholly in front of the center. The posterior sinus 

 is less than a fourth of the abdomen's length; the anal laminae are 

 lateral, but much nearer the distal than the proximal end of the 

 sinus; the posterior lobes are broad and bluntly rounded. 



The swimming legs are like those of the adult except that they 

 are relatively longer and more slender. The boot-like lobes at the 

 base of the posterior pair are also longer and narrower. Such 

 great differences are suggestive of another species, but the .ap- 

 pendages and mouth-parts are exactly like those of the adult, and all 

 the intermediate stages are present to show^ just how the trans- 

 formation progresses. 



Young female. — The young female differs from the adult in much 

 the same particulars as the male. The abdomen of one only 4 mm. 

 in length is seen in Figure 7, and it may be noted that it corresponds 

 exactly with that of the young male. Two or three details stand 

 out much more clearly than in the adult. The first of these is the 

 peculiar form of the basal plate of the maxillipeds shown in Figure 

 8. Neither Thiele's figure (pi. 8, fig. 68) nor that of the present 

 author ^ show the sharp bend in this plate just above the raised area. 



» Proc. U. S. Nat. Mus., vol. 42, 1912, pi. 30, flg. 3. 



