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 bod 
are very remarkable. These organs not only move on their respec- 
tive sternums, 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 
sete, or pinnule, similar to those terminating the abdomen, which 
renders them well adapted for swimming. 'he cilixw, though 
very long, are exceedingly slender. The pinnule 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 ; 
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 set, 
or pinnule, whose length about equals the preceding joint of the 
leg. The cilia are very short on the outer side of these pinnule ; 
but of the usual length on the inner. At the apex there are four 
short obtuse naked sete. The sternum to which this pair of legs 
is articulated, is very narrow and terminates on each side in @ 
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 sé 
joint. The basal is very short, and has on its posterior margin 4 
