236 Description of a Species of Caligus. 



3. Anterior Thoracic Segment. — The first two pairs of legs on 

 this segment are natatory, the third is expanded into a broad apron. 

 Preceding the first pair of these legs, there is, on the medial line, a 

 broad furcate corneous process, directed backward, and capable of 

 being elevated or depressed. 



The articulations of the two pairs of natatories with the body 

 are very remarkable. These organs not only move on their respec- 

 tive siernums, with which they are articulated, but the sternums 

 have a hinge motion on their posterior margin, in which the legs 

 participate. For this purpose the basal joints are attached to the 

 adjoining parts of the venter by the greater part of their anterior 

 side. The whole distance between the apices of the basal joints 

 in the first pair of natatories, forms thus a single hinge on 

 which the legs revolve ; and in the second pair of natatories, the 

 greater portion of the second joint is similarly attached, and for 

 the same purpose. 



Several of these joints are provided with long, finely ciliated 

 setce, or pinnulae, similar to those terminating the abdomen, which 

 renders them well adapted for swimming. The ciliae, though 

 very long, are exceedingly slender. The pinnulse appear to be 

 mostly hollow. They contain a central longitudinal line, which 

 appears on the first view to divide them into two portions ; further 

 examination has led us to believe that this is not the case. 



The first pair, are composed of three nearly cylindrical joints, 

 the first two of which are very similar in size, and the third about 

 one half the length of the preceding. The basal joint has a short 

 movable hairy seta at its apex, and another on its posterior margin j 

 also a very short jointed appendage on the same margin near 

 its extremity. The second joint has a similar seta near its apex 

 and is ciliated on its posterior margin. The terminal joint is fur- 

 nished on its posterior margin with three long finely ciliated setEe, 

 or pinnules, whose length about equals the preceding joint of the 

 leg. The cilise are very short on the outer side of these pinnulee ; 

 but of the usual length on the inner. At the apex there are four 

 short obtuse naked setae. The sternum to which this pair of legs 

 is articulated, is very narrow and terminates on each side in a 

 process lengthened posteriorly for articulation with the legs. 



The second pair of natatories are composed of five stout com- 

 pressed joints, with a large tri-articulate appendage to the second 

 joint. The basal is very short, and has on its posterior margin a 



