576 
BULLETIN OF THE BUREAU OF FISHERIES 
full growth. However, for the upper pallial lobe the case is different. Specimens a 
centimeter or slightly greater in length have about the same numbers of ocelli below 
and above. The ocelli of the upper lobe continue to increase until (with 40 to 55 
formed in those I examined) they are about one-fourth to one-half more numerous 
than those of the lower lobe. The ocelli of the ear section, rather small and few in 
number (about a dozen, chiefly at the posterior ear), are of interest because they are 
exposed when the shell is closed. Dakin (1909) noted that in various species the 
ratio of the number of eyes on the upper lobe to the number on the lower lobe increased 
with the relative flatness of the upper valve, but that even if the upper valve was the 
more convex the upper lobe had more eyes than the lower. 
The abundance of scallop eyes and the unequal distribution between the two 
mantle lobes have led to considerable discussion. Patten (1886) has queried the 
scallop’s need for many organs of vision if two suffice for other forms. Dakin be- 
lieved numerous eyes were needed because they were not movable in various directions 
and images would be formed only of objects directly in front of an eye. A reason- 
able explanation for the greater abundance of eyes on the upper lobe seems more 
difficult to find. It is perhaps worthy of notice that this uneven distribution tends 
to equalize the light perception of the two valves. 
The ocelli of Pecten have been the object of much study since Poli (1795) sketched 
their external appearance, recognized their resemblance to the vertebrate eye, and 
named some of the parts. Garner (1837) stated that scallops possess “small, brilliant, 
emeraldlike ocelli, which, from their structure, having each a minute nerve, a pupil, 
a pigmentum, a striated body, and a lens, and from their situation at the edge of the 
mantle, where alone such organs could be useful, and also placed, as in Gasteropoda, 
with the tentacles, must be organs of vision.” Krohn in 1840, according to Sharp 
(1884) and Dakin (1910), greatly advanced knowledge of the structure of the ocellus 
which he designated a closed vesicle. Apparently he was the first to note the septum 
and the division of the nerve into two branches. 
The “modern” period in the study is taken to begin with the paper by Hensen 
(1865), who not only advanced knowledge of eye structure but also proved something 
of a seer when, after remarking upon the clearness with which details of the eye may 
be observed, he asked “but how much toil (Muhe) will be necessary before the entire 
structure of this cubic millimeter will be understood?” In the many years that have 
elapsed a great amount of minute attention has been given the structure of this di- 
minutive organ and presumably the end is not yet. 
Among later accounts may be mentioned Hickson (1880), Patten (1886), Rawitz 
(1888), Schreiner (1896), Hesse (1900, 1902, 1916), Hyde (1903), Dakin (1909, 1910a), 
and Kiipfer (1916). Of these only Hyde worked with P. irradians or other American 
scallop. Some of her findings were so different from those of other investigators that 
Dakin (1910a), working with European species, made special but unsuccessful efforts 
to confirm them. The retina evidently is very complicated and the chief cause of 
disagreement. The paper by Dakin (1910a) presents a clear, detailed, and very 
useful account based on extensive personal investigations and with a careful survey of 
the literature. The book by Kiipfer (loc. cit.) is unusually elaborate and complete. 
Notable features are additions to the knowledge of the outer layer of the retina and of 
the development of the eyes, and a detailed comparative anatomical discussion. 
The recent account (Light, 1930) of light receptors in My a is of interest. 
I have examined several eyes fixed in formalin, stored in alcohol, and cleared in 
glycerin. Although histological details were not determinable, major structures 
