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 extraordinary 

 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 antennae 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 form, being about ^jVo 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. 



Plate III. 



Fig. 1. Under view of a male, exhibiting the various organs, and the muscles 

 that move them. A, minute papillae, supposed to correspond to inner antenntE ; I, 

 a cup, for the attachment of the animal ; L, antennae ; 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. 



Vol. XXXIV.— No. 2. 34 



