VII EYES IN BIVALVE MOLLUSCA 1 89 



Lorica^ and IschnocJdton} None of our English species appear 

 to possess them .2 



Eyes in Bivalve Mollusca. — Some, possibly most, of the 

 Pelecypoda possess, in the larval state, true paired eyes at the 

 oral end of the body. These become aborted as the animal 

 develops, since that part of the body becomes entirely screened 

 from the light by the growth of the shell. To compensate for 

 their loss, numerous ocelli^ or pigmented spots sensitive to the 

 action of light, are in many cases developed on different parts of 

 the mantle, functionally corresponding to the ' eyes ' of Chiton 

 described above. As in Chiton^ too, we have here an interesting 

 series of instances in which true eyes have suffered total obliter- 

 ation, through disuse, and, as if to restore to the animal in some 

 measure its lost sense, visual organs of a low power have sub- 

 sequently been developed and are now observed in various stages 

 of specialisation. 



Concentration of Eyes in Special Parts of the Mantle. — 

 Sharp has shown ^ that in several species of 0)>trea, Cardium^ 

 Anomia, Lima, Avicula, Area, and Telllna pigmented cells, with 

 a highly refractive cuticle, are scattered over a considerable 

 portion of the mantle. Experiment has proved the powers of 

 'vision,' i.e. of sensitiveness to different degrees of light, pos- 

 sessed by these organs. In Dreissena polymorpha. Tapes deeus- 

 satus, and two species of Venus these cells are concentrated on 

 that particular part of the mantle which is not always covered 

 by the shell, i.e. the siphon, but since the siphon can be com- 

 pletely retracted within the shell, there is no special provision 

 for their protection. A further step is shown in the case of 

 3Ii/a arenaria, where the siphon is scarcely capable of complete 

 retraction. Here, while some of the pigment cells are scattered 



1 The nature of the grouping of the eyes into rows varies considerably in 

 different species. As a rule, the rows radiate from the beak, but occasionally 

 they run parallel to the girdle. In Tonicia Uneolata Fremb., they are grouped, 

 as it were, under the shelter of strongly marked longitudinal wavy lines. 



2 Shell-Eyes in other Mollusca. — The Rev. J. E. Tenison- Woods (Trails. 

 Linn. Soc. N. S. Wales, xxii. p. lOG) is of opinion that ' shell-eyes ' are by no means 

 confined to the Chitonidae, but that, in fact, multiplicity of eyes of this kind is 

 the rule rather than the exception among the Mollusca. He finds (1) exceedingly 

 minute and numerous ' eyes' on the outer surface of the shell in both univalves 

 and bivalves ; (2) large and solitary ' eyes ' in the shell substance ; (3) eyes on 

 the mantle lobes in both univalves and bivalves ; (4) eyes on the opercula. 



3 Mitth. Stat. Zool. Xtap. v. p. 447 ff. 



