Description of a Species of Caligus. 265 
also evident from the nature of the organs that follow. The space 
contained within the U suture in the Argulus is the analogue of 
the much larger and more distinctly separated segment, which we 
have called the cephalic in the above description. The anterior 
abdominal joint of the Caligus is wholly wanting in the Argulus ; 
and the valves in the circulation which occupy the posterior tho- 
racic joint, far from the extremity of the body, have an analo- 
gous situation in the Argulus, close to the last joint of the body. 
This joint being small in the Argulus forms a very distinct and reg- 
ular heart, and serves to keep up a much more active circulation 
than in the Caligus, where the corresponding part is large and 
less energetic in its action. It is remarkable that the circulation 
in the two should be the reverse in almost every particular; the 
ventral current instead of being upward in the Argulus runs to- 
wards the terminal joint of the body; instead of meeting from 
. the two sides in front and returning down the medial line, it goes 
out in two currents near the medial line and returns in the wings 
of the shell. This however will not appear so extra 
when we consider that the animals are the reverse of one another 
in some particulars. The cephalic segment in the Caligus is very 
large and broad, and there is therefore space for the current fur- 
nished to the antenne and cephalic organs, to flow along the sides, 
and return along its centre; but in the Argulus, this portion is so 
small that there is only room for the out-current, and the blood is 
compelled to turn outward into the wings of the shell or thoracic 
portion, which is very much larger than in the Caligus, The cur- 
rents are much more definite in their limits in the Argulus, and 
more uniform in their velocity and course; the particles of the 
blood are also less variable in size and Sarin being about ;;'55 of 
an inch, in length. The organs of the mouth are also similar in 
position and in the form of the mandibles. This analogy might 
be traced much farther; but we reserve further remarks for a fu- 
ture occasion. 
EXPLANATION OF THE PLATES. 
Prare III. 
g. 1. Under view of a male, oon the various organs, and the muscles 
Pl move them. A, minute papille, sw d to correspond to inner antenne ; I, 
a cup, for the attachment of the animal ; gi antenne ; d, one of the muscles mov- 
ing the mandibles. ‘The arrows point out the course of the blood. 
Fig. 2. Back view of male, natural size of one of the largest individuals. 
To... SX XIV.—No. 2. 34 
