Description of a Species of Caligus. 249 



2. A lens, simple, spherical, and distant from the cornea; its 

 diameter is about half that of the cornea :* 



3. A colorless, transparent fluid, which we presume to be the 

 aqueous humor, occupying the space just within the cornea : 



4 A deep red, nearly black pigment, which forms the colored 

 spot supporting the eyes. 



We have not observed the vitreous humor. In the spherical 

 form of the lens, the eyes resemble the same organs in fishes. They 

 are not movable, and have no connections except by the optic 

 nerve. The adjacent parts are transparent, enabling the animal 

 to see in both directions. We have already described the shell 

 above, and referred to its representation in fig. 8, PL IV. It is 

 perfectly flat, without any spherical projection, corresponding to 

 that of the cornea within. The translucent elongated space in fig. 

 8j lies in the shell, and passes over the space between the eyes. 



b. The nerves. 



The nervous system contains but two ganglions, and these by 

 their close approximation appear at first, to compose but one. 

 They are situated directly behind the eyes, the one above the 

 esophagus, and the other below it, and are so intimately connect- 

 ed on each side of this portion of the alimentary canal, that it has 

 been found impossible to separate them, (fig. 20. ) Indeed, it would 

 scarcely convey an incorrect idea of the form, to describe it as a 

 single mass, with a longitudinal cavity through the centre, for the 

 passage of the esophagus. The size of the united ganglions is 

 rather greater than that of the buccal mass. The nerves arising 

 from these ganglions are flat, fibrous cords, enclosed within a 

 membranous envelope or neurolemma. This neurolemma is often 

 one fourth wider than the bundle of nervous fibres contained with- 

 in, and these fibres appear to pass through without any attach- 

 ment. The neurolemma is sometimes slightly folded, which 

 gives a crenated appearance to the margin of the nerve. 



The brain or cephahc ganglion has a broad ovate or sub-cordate 

 form. It gives ofi" three pairs of nerves. 



The first pair, (a fig. 20, PI. lY,) leaves the central part of the 

 anterior margin and passes directly to the eyes. As the eyes are 

 adjacent to the ganglion, these nerves are very short. 



* The lens in the simple eyes of Crustacea is usually described as being in im- 

 mediate contact with the cornea : it was very evidently distant from this mem- 

 brane, in the Caligus. 



Vol. XXXIV.— No. 2. 32 



