96 



borne tentacles, and on the median and smaller are 

 situated the eyes (fig. 4, E.), 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 maximus the numbers average about 35 on 

 the left (fiat) side and as few as ten, separated by 

 long intervals, on the right (convex) side. In Pecten 

 oi)ercularis the numbers are more equal, the following 

 being some series: — right, 37, 35, 41, 51; left, 52, 51, 56, 

 61 ; but it will be seen they still are more numerous on 

 the left than on the right, though the left valve of Pecten 

 maximus is the flat one, whereas the right valve is the 

 flatter of the two in Pecten oyevcularis. 



Patten (35) observed that P . jacohaeus always lay on 

 its right valve, that is the convex, and if turned over it 

 soon righted itself. 



Pecten maximus, which like P. jacohaeus 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. maximus. One 

 might at first expect that the eyes looking up toward the 

 light would be the better developed, but the eyes on the 



