ECONOMIC RELATIONS, ANATOMY, AND LIFE HISTORY OF GENUS LERNyEA. 1 87 



THE FOURTH COPEPODID LARVA. 



A large number of both sexes of this stage (fig. 53-61, pi. xii) were obtained from 

 the gills of the short-nosed gar, thus completing the larval development, since during 

 this period both kinds of larvae become sexually mature and mating takes place. 



The female larva. — This larva has greatly elongated and is now 1.25 to 1.50 mm. 

 in length, including the anal setae. The cephalothorax is somewhat oval in shape, being 

 narrowed anteriorly, and its longitudinal and transverse diameters are in the propor- 

 tion of 8 to 5. No new thorax segment has been added, but the genital segment has 

 greatly increased in size, and there are now three abdominal segments instead of two. 



The first antennae remain four- jointed; the second pair have become longer, and 

 the prehensile claw at their tips has increased greatly in size. Of the mouth parts the 

 mandibles and first maxillae can be made out much more easily than in the adult. The 

 first four pairs of swimming legs have three-jointed rami, and each fifth leg now has 

 a single distinct one-jointed ramus tipped with long setae. 



At the posterior corners of the genital segment is a single large seta on either side, 

 marking the sixth legs. The fifth segment is fused with the genital segment, the sepa- 

 ration being indicated by deep lateral notches behind the bases of the fifth legs. 



In a little older larva there is a marked elongation of the thorax, the segments 

 separating from one another so as to leave wide lateral notches. (See fig. 54, pi. xii.) 



Female larvae of another species in the fourth copepodid stage were obtained 

 from the gills of the bullhead, Ameiurus nebulosus, from Drury Slew, July 23, 1915. In 

 these the carapace and the free thorax segments were relatively shorter and wider (fig. 

 41-49, pi. xi); the fifth segment was distinctly separated from the genital segment, the 

 latter being the wider of the two and nearly twice as wide as long; and the anal laminae 

 and setae were also longer and narrower. 



The fifth legs had a wider basal joint, armed with a stout spine on the outer margin, 

 while the ramus was armed with three spines, two at the tip and one on the outer mar- 

 gin; no rudiments of the sixth legs were visible in dorsal view. In the maxillipeds the 

 terminal joint was relatively shorter and narrower, and the basal joint was much less 

 inflated through the center. It is impossible to locate this species with certainty, but 

 it is at least highly improbable that it is a larval form of tortua, the adult female of which 

 is sometimes found upon this same bullhead. It is also worthy of note that its host, 

 the bullhead, is a fish which stays on or close to the bottom, while the short-nosed 

 gar, the host of the variabilis larva stays close to the surface of the water. 



The male larva.^This larva is similar to the female in most particulars, but shows 

 some sex differences. The body in general is shorter and relatively wider, especially 

 the cephalothorax, whose longitudinal and transverse diameters are in the proportion 

 of 7 to 6. The free thorax segments are also wider and shorter than in the female 

 (See fig. 36, 53, pi. X, XII.) 



The only differences in the appendages are in an increase in the size and curvature 

 of the prehensile claws on the second antennae and the terminal claws on the maxilli- 

 peds, longer and stouter rami on the swimming legs, and in the size of the sixth legs. 



Male larvae of another species, probably cruciata, were obtained from the gills of the 

 Sanger. (See fig. 36, pi. x.) On these the cephalothorax is relatively longer and narrower, 

 but the anterior margin is wider and projects some distance over the bases of the 

 antennae. The second (first free) thorax segment is as wide as the carapace, but the 



