170 WILLIAM J. DAKIN 
Eyes are to be found of very varying form and complexity 
of development. In a great many cases an open cup-shaped 
retina is to be seen (resulting from ectodermal invagination), 
but there is no lens, cuticular or otherwise. ‘The retina in nearly 
all cases consists of rod-cells bearing rods which are directed 
distally. In a large number of the eyes, the histology of which 
has been investigated, the details are not very similar to the 
Kye of Peripatus. Hesse’s figure of the eye of Sipho- 
nostomum diplochaetos is, however, curiously like that 
of the early illustrations of the Peripatus eye so far as the retina 
TEXT-FIG. 3. 
Diagram of lens and corneal layers of eye of Polychaete (Vanadis 
formosa), modified after Hesse. Note similarity of arrange- 
ment of layers to that found in Peripatus. C.o. = outer cornea ; 
Ci. = inner cornea; Cu. = cuticle; Hy. = hypodermis; L. = lens; 
R.=retina (structure not shown). 
is concerned. Both the vertical sections and those taken in 
the plane of the retima indicate this, and no doubt the structure 
is almost exactly the same as that of the Eye of Peripatus. 
A detailed re-examination with up-to-date methods would be 
necessary to make it certain. 
The remaining features (dioptrical) of this Polychaete eye 
are quite unlike those of Peripatus. The eye is not nearly so 
well developed as that of the latter. 
One of the best-developed Polychacte eyes is dame in the 
group Alciopidae. We have here a vesicular eye (see Text- 
fig. 3) with enclosed and well-developed lens. There are 
many resemblances to the Eye of Peripatus. The cuticle, for 
