ECONOMIC RELATIONS, ANATOMY, AND LIFE HISTORY OF GENUS LERN^A. 1 85 



joint without plumose setae, but armed with 8 or 10 long spines. Second antennae and 

 mandibles as in the nauplius, except that the endopod of the second antennae is now 

 tipped with a single plumose seta, and three large spines on the posterior margin. 



Behind the mandibles on the dorsal surface of the body there is a single large spine 

 on either side close to the lateral margin. 



Toward the posterior end the swimming legs can be seen partly differentiated 

 inside the body. 



At the posterior end are two large anal laminae, each armed with three plumose 

 setae, the two inner ones the largest, and the two central ones the smallest. 



A compound eye is now distinctly visible close to the anterior margin, and just 

 in front of it that margin projects slightly. 



There is no pigment on this metanauplius, and the internal anatomy, with the 

 exception of the eye, is still wholly invisible. 



Total length, 0.25 mm. Width, 0.125 ™ni- 



This metanauplius molted once within a few hours, and at the second molt 

 passed into the first copepodid stage some time during the third day. 



THE FIRST COPEPODID LARVA. 



With the molt from the metanauplius into the first copepodid stage (fig. 31-35, 

 pi. x), the larva ceases its free-swimming life and enters upon its first period of para- 

 sitism. It seeks out a temporary host, upon whose gills it can continue its development 

 up to sexual maturity and fastens itself to the outside surface of the gill filaments by 

 means of its second antennae and maxillipeds. While this attachment is strong enough 

 to hold the larva securely in place even when the gill is rinsed off under a faucet, it 

 can still be easily loosened at the pleasure of the larva. Indeed, these copepodid larvae 

 are much the most lively parasites of all the copepods infesting fish. It is practically 

 impossible to catch one of them alive without injuring it, for upon being touched they 

 let go of the filament, dart about swiftly in the water, and then seize the gill in another 

 place. But they can often be deceived by cutting off the entire filament and removing 

 it while they still chng to it. Having obtained one in this way from the gills of the 

 short-nosed gar, Lepisosteus platostomus, we may examine its structure. It is the larva 

 of Lerncea variabilis . (See fig. 31.) 



The cephalothorax is elliptical in shape, its longitudinal and transverse diameters 

 being in the proportion of 4 to 3. It is somewhat enlarged through the bases of the 

 first antennae and the anterior and posterior margins are nearly straight. It is fol- 

 lowed by three free thorax segments, which diminish regularly in width, but are of 

 about the same length. Finally, there is a rectangular segment, longer and narrower 

 than the last free thorax segment, which represents the fused abdominal segments. 

 It bears two large anal laminae at its posterior comers, each armed with a very long and 

 jointed inner seta and four much shorter outer ones, which diminish in size regularly 

 outward. 



The first and second antennae are the same size and each is two- jointed, the terminal 

 joint a little smaller than the basal and heavily armed with setae. In addition to the 

 setae the second pair have a strong prehensile claw at the inner distal comer, and they 

 share with the maxillipeds the task of holding the larva in place upon its host. 



The mandibles, maxillae, and maxillipeds are the same in all particulars as those 

 of the adult. Each swimming leg of the two pairs is made up of a short and wide basal 



