248 Description of a Species of Caligus. 



of the remaining two, (x,) arises just above the posterior sinus, 

 and the other from the inner margin of this sinus ; both are at- 

 tached on the back, and inserted near the articulation of the ster- 

 num. The flexor muscles arise below, just outside the apron, and 

 occupy the greater part of its interior. A single muscle is attached 

 near the articulation of the sternum, and passes into the basal 

 portion. 



This apron, appended to the cephalo-thoracic segment, forms 

 the anterior portion of the body into a large, broad cup, which is 

 perfectly closed, with the exception of a small opening at each of 

 the posterior sinuses. These we have already described as pro- 

 vided with a folded membrane, furnished with muscles capable of 

 drawing it over and completely shutting the opening. The mem- 

 branous margin of the animal near the antennae, has also a fold 

 by which a small leak, if it be such, is closed. Considering these 

 several provisions, it is probable, that the whole of this anterior 

 portion of the animal is especially adapted to enable the animal 

 to attach itself firmly during the rapid motions of the fish, and 

 that the small marginal cups in front are relied on, only while the 

 fish is stationary, or but slowly moving. 



The remaining pair of legs are moved by short slender muscles, 

 and seem to possess little power. They usually hang loose and 

 motionless while the animal is swimming, and when attached to 

 the body of the fish, are commonly extended by the side of the 

 abdomen. 



III. Nervous System. 



a. The organs of the senses. 



The only organs in the Caligus, which we have been able to 

 distinguish as the undoubted residence of special senses, are the 

 eyes and the antennae. The latter organs have already been de- 

 scribed ; it remains to explain the structure of the eyes. 



The eyes are wholly internal^ and are situated near the centre 

 of the posterior cephalic segment, directly over the lower part of 

 the buccal mass. They are two in number, simple in their struc- 

 ture, and placed near one another, on a single reddish-black ground. 

 They project from each side of this colored ground, with a spher- 

 ical surface, somewhat exceeding a hemisphere. On dissection 

 we readily distinguish the following parts. 



1. A cornea^ which is thin and transparent and forms the spher- 

 ical surface of the eye : 



