vit EYES IN BIVALVE MOLLUSCA 1 89 



Loi'ica, and Ischnochiton} None of our English species appear 

 to possess them.'^ 



Eyes in Bivalve MoUusca. — 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 obliteration, 

 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 Ostrea, Cardium, 

 Anomia, Lima, Avicula, Area, and Tellina 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, possessed 

 by these organs. In Breissena jpolymorpha. Tapes decussatus, and 

 two species of Venus these cells are concentrated on that particu- 

 lar part of the mantle which is not always covered by the shell, 

 i.e. the siphon, but since the siphon can be completely retracted 

 within the shell, there is no special provision for their protection. 

 A further step is shown in the case of 3fi/a are7iaria, where the 

 siphon is scarcely capable of complete retraction. Here, while 

 some of the pigment cells are scattered about over the surface of 



^ 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 par- 

 allel to the girdle. In Tonicia lineolaia Fremb., they are grouped, as it were, under 

 the shelter of strongly marked longitudinal wavy lines. 



- Shell-Eyes in other Mollusca. — The Rev. J. E. Tenison-Woods {Trans. Linn. 

 S<jc. A'. *S'. Wales, xxii. p. 106) is of opinion that 'shell-eyes' are by no means con- 

 fined to the Chitonidae, but that, in fact, nuiltiplicity 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 opercida. 



» Math. Stat. ZooL Neap. v. p. 447 fl'. 



