428 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
borne tentacles, and on the median and smaller are 
situated the eyes (fig. 4, H.), on short stalks which 
resemble the basal portions of the ordinary tentacles. 
Many workers have noticed this similarity, which, 
together with the course of the nerve fibres, has led them 
to the conclusion that these are homologous organs 
modified for different functions. The Pecten eye is an 
inverted eye like those of Planarians and Vertebrates, 
and resembles the latter in some respects. It is derived 
from both ectoderm and mesoderm, the lens being formed 
from the latter. ‘The number of eyes varies considerably 
in the different species, and there is further a difference 
in the number on the two mantle lobes. 
In Pecten maavimus the numbers average about 55 on 
the left (flat) side and as few as ten, separated by 
long intervals, on the right (convex) side. In Pecten 
opercularis the numbers are more equal, the following 
being some series :—right, 37, 35, 41, 51; left, 52, 51, 96, 
61; but it will be seen they still are more numerous on 
the left than on the right, though the left valve of Pecten 
maaimus is the flat one, whereas the right valve is the 
flatter of the two in Pecten opercularis. 
Patten (35) observed that P. jacobacus always lay on 
its right valve. that is the convex, and if turned over it 
soon righted itself. 
Pecten maaimus, which like P. jacobaeus has very 
unequal valves, is always to be found lying on the right 
convex valve-—which suggests that the greater develop- 
ment of eyes is always on the flat upper valve. This is 
supported by the condition in P. opercularis, where little 
difference can be detected between the valves, and the 
number of eyes is more alike than in P?. maeimus. One 
might at first expect that the eyes looking up toward the 
light would be the better developed. but the eyes on the 
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