78 BULLETIN OF THE 
this lens, and in fact covering much of its proximal face, is a rather 
ifregular mass of cells, the retina. In the living animal the cells of the 
retina contain a great quantity of black or reddish black pigment. This 
coloring matter, however, is so readily soluble in alcohol, that in speci- 
mens preserved in that fluid all traces of it disappear. The optic nerve 
(n. opt., Fig. 18), an imperfectly defined bundle of fibres, emerges from 
the retina near its posterior dorsal edge, and passes directly backward to 
the brain. 
The lenses of the two lateral eyes in Pontella are so near each other 
that their median faces are almost in contact (compare Plate III. Fig. 
29). The retinas of the two eyes, as Claus (’63, p. 47) has observed, 
are united with one another on their median faces, and so intimately 
that they are apparently incapable of independent motion. 
The two retinas together may be rotated on their lenses through an 
angle of about forty-five degrees. , The plane of this rotation corresponds 
to the sagittal plane of the body, and the rotation is accomplished by 
two pairs of muscles, one for each retina (compare Claus, 763, Taf. IIT. 
Fig. 6). One pair of these muscles is shown in Figure 18. They occupy 
a plane approximately parallel to the sagittal plane of the body, and 
the effects of their contractions must be apparent from their positions, 
When both muscles are relaxed, the retina occupies a position substantially 
as shown in Figure 18. By the contraction of the posterior muscle, tite 
retina may be drawn upward and backward over the surface of the lens, 
till its axis, instead of pointing dorsally, is directed forward and upward 
at an angle of about forty-five degrees with its original position. The 
retina is not usually held for any great length of time in this position, but 
is soon returned by the contraction of the anterior muscle to its normal 
place. The backward motion of the retina is accomplished with such 
rapidity that the animal has the appearance of winking. The forward 
motion is rather slower. 
Each lens in Pontella is composed of concentric laminze (Plate III. 
Fig. 29, Ins.). A considerable portion of its distal surface is intimately 
connected with the superficial cuticula (Plate II. Fig. 18), although a line 
of demarcation between lens and cuticula can always be distinguished. 
When the anterior half of the body of Pontella is boiled in a strong 
aqueous solution of potassic hydrate, and afterwards subjected to the 
action of concentrated nitric acid, all. the soft parts are dissolved, and 
only the very resistant chitinous structures remain. In specimens 
treated in this way, the lenses retain their firm connection with the 
superficial cuticula, and differ in appearance from those in the living ani- 
