78 BULLETIN OF THE 



this lens, and in fact covering much of its proximal face, is a rather 

 irregular 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 

 («. 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, (me for each retina (compare Claus, 'G3, Taf. III. 

 Fig. 0). 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, the 

 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 laminae (Plate III. 

 Fig. 29, lux.). A considerable portion of its distal surface is intimately 

 connected with the superficial cuticula (Plate II. Fig. IS), 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- 



