468 Benjamin Sharp 



forms, while much more so in Solen^ Mactra etc. wliere the pigmented 

 cells are in grooves at the base of the tentacles and protected by them. 



Now the simplest form of organ capable of vision must bave pig- 

 ment to absorb the light and a refractory body , as is exemplified in 

 the lowest known form of visual organ, the eye spot oi Euglena viridis, 

 a flagellate infusorian ; bere only a part of a cell acts as an organ of 

 vision. 



In Ostrea the conditions necessary for sight are present, a number 

 of cells that contain pigment and bave as their external surface a highly 

 refractive body , the transparent cuticle , and that vision i s present is 

 proven by experiment i ; further the fact that these cells are placed only 

 where they could be of use to the animai. Morphologically eyes should 

 be at the oral extremity of the animai , where we do find them in 

 the larvae of some of the LamellibrancMata ^ ; but the organs of vision 

 found in the adult are only physiologically eyes — adaptive or ceno- 

 genetic organs. 



They are generalized and scattered more or less on the whole 

 mantle, but principally at the posterior end , this we find to be more or 

 less the case in all the Asiphonata. When a higher stage is reached, 

 and a siphon begins as in Dreissensia polymorplia , we bave the cells 

 concentrated on that part of the mantle 'siphon), which extends farthest 

 from the shell. In Dreissensia and Venus where, when the animai is 

 disturbed it retracts the siphon wholly within the shell . we bave the 

 pigment cells in spots on the surface of the siphon without any parti- 

 cular protection. In J/^/a, however, where the siphon can hardly be 

 retracted within the shell, we have the pigment cells protected by being 

 placed in grooves at the base of the tentacles, where in the rapid with- 

 drawal of the siphon through the sand , in which the animai lives no 

 injury to the delicate organs takes place. 



A step higher we have the state of aflfairs in Solen and Mactra, 

 when the pigmented grooves are only at the bases of the tentacles, 

 and the grooves more or less specialized in number and complexity. 

 A more highly developed or specialized type is Patella^ where the 

 grooves are specialized to a spherical pit, but stili open and bathed 

 with the surrounding medium. 



These are probably the steps taken in the phylogenetic develop- 

 meut of the eye , first general , then specialized to that part of the 



1 J. A. Ryder, Primary Visual Organs etc. 1883. 



2 C. Gegenbaur, Elem. of Comp. Anat. etc. 18TS. § 272. p. 353. 



