NATURAL HISTORY OF THE BAY SCALLOP 
577 
could be distinguished. Figure 6, showing only the general structure, is based on 
dissection of such material. 
The following brief account is based largely on that of Dakin: 
The eyes are situated at the end of short stalks located among the tentacles of 
the middle fold. The stalk is composed chiefly of connective tissue and contains 
muscle fibers, large blood spaces, and the optic nerve. This nerve has been supposed 
to come from the ganglionic circumpallial nerve, but according to Dakin (1910a) 
most of its fibers connect directly with the visceral ganglion. The epithelium is 
pigmented around the eye forming what has been termed an iris (Patten, 1886). 
In front of the lens it is clear and is termed cornea. 
Separated from the cornea by a layer of clear connective tissue is the lens, which 
is composed of many transparent cells of unusual shape and arrangement. The inner 
face of the lens is much more convex than 
the outer. Back of the lens and overlying 
the retina is the membranous septum. 
The complex, inverted retina has been 
considered to consist of various numbers of 
layers but, according to Dakin (1910a), is 
best considered as of two, an outer layer of 
distal sense cells and an inner one of rod 
cells and rods. The outer layer is innervated 
by the outer or distal branch of the optic 
nerve which enters from the front through 
the septum, the inner layer by the inner 
or proximal branch which enters through 
the periphery. 
Back of the retina are two prominent 
layers, each of which has been termed 
“tapetum.” Patten (1886) referred to the 
inner or frontal of these, which is refractive 
and gives the eye its metallic glitter, as the 
argentea — an appropriate name. However, 
this layer previously had been termed 
“tapetum” by Krohn who discovered it 
(Dakin, 1910a). The outer or abfrontal of 
these two layers was designated “tapetum” 
by Patten but probably is best referred to as pigment layer (Dakin, loc. cit.). The 
layer of connective tissue surrounding the abfrontal half of the optic vesicle — that is, 
that portion back of the septum — was termed “sclerotica” by Patten, to which some 
authors have objected. 
Although there has been so much work on the ocelli, most of it has been histo- 
logical so that knowledge of their functioning still is rather unsatisfactory. From, 
their structure they appear to be organs of vision. However, their very elaborate- 
ness and high development only add to the puzzle, for if such highly organized 
structures are organs of sight, their vision would be expected to be excellent and pot 
only easily demonstrable but evident beyond a doubt. On the contrary, although 
scallops undoubtedly are sensitive to light, any image vision they possess is so poor 
or so limited as to be difficult to establish. One difficulty is in obtaining specimens 
that react normally (Wenrich, 1916, states that only scallops from very shallow 
Figure 6.— Sketch of eye on dissection of unstained mate- 
rial cleared in glycerin. C, Cornea; /, iris, made up of 
pigmented and clear layers; L, lens; 0. N. /., inner 
branch of optic nerve which spreads around the base 
of the optic vesicle to the edge of the retina; O. N. O., 
outer branch of optic nerve, the latter course of which, 
leading to the face of the retina, was not followed in 
these dissections; R, retina overlain by septum (not 
designated) with three noticeable layers (an outer or 
outer ganglionic layer, a layer of rod cells, and an inner 
layer of rods); P, pigmented layer (tapetum of Patten); 
T, tapetum (argentea of Patten) 
