AMERICAN PARASITIC ARGULIDM-WILSON. 679 



!ind then curve inward toward each other, sometimes almost meeting 

 at the median line, and finally diverge again toward the bases of the 

 anterior antennae 



Each one thus takes somewhat the shape of the old-fashioned letter/", 

 the two letters being placed back to back. In foUaceiis the anterior 

 end of these ribs appears branched (Claus) as also in salminl (Kroyer), 

 but this occurs in none of the North American species examined. 



These long ribs are jointed once just behind the brain, to give the 

 anterior region of the head greater freedom of motion. 



They are also connected with each other, posterior to the joints, by 

 several cross ribs, the first of which is close to the joints and quite 

 concave toward the brain. 



In some species {niger, foUaceiis^ fundul!) these cross ribs give the 

 appearance of vertebra, especially in alcoholic specimens, as noted by 

 Claus, but they are practically invisible in the living animal by reason 

 of their transparency, as can be seen in the photographs. Other stout 

 branch ribs extend radially outward from the mid-line toward the 

 .lateral margin, the largest and most conspicuous pair being situated 

 between the posterior niaxillipeds and the anterior swimming legs (figs. 

 14, etc.). 



In addition to these ribs there are two oval chitinous rings in each 

 lateral ala at some little distance from the margin. 



The anterior one is short, often triangular, and situated about 

 opposite the mouth, while the other is much larger and longer and 

 reaches nearly the entire length of the lateral area. 



The shape and arrangement of these lateral rings is peculiar for each 

 species and affords a very reliable secondary specific character, as will 

 be readily seen by comparing any two of the figures showing the ventral 

 surface. 



The free thorax has a flattened conical form, the segments dimin- 

 ishing in size from in front backward. The posterior segment is tri- 

 angular in outline, the apex of the triangle terminating posteriorly 

 on the ventral surface at the opening of the oviduct or the vas 

 deferens. On the dorsal surface this posterior segment is usually 

 more or less overlapped by the abdomen. Each segment, including 

 the one fused with the head, carries a pair of swimming legs, the pos- 

 terior pair pointing obliquely backward in consequence of the oblique 

 sides of the segment which carries them. In alcoholic specimens these 

 posterior legs are often curled in and wholly covered by the abdomen. 

 (See figure of A. nlger.) The thorax has no other appendages save in 

 purpureus, where there are two scale-like lobes attached to the poste- 

 rior segment on the ventral surface and projecting backward beneath 

 the abdomen (fig. 65). 



The latter is much thinner than either of the other tw^o body regions. 

 It consists of little more than two plate-like lobes or lamella% filled 



