95 



With reg-ard to the function of this well-developed 

 organ, it is difficult to see that it can be any other than 

 a water-testing', either smell or taste organ, yet this sense 

 is generally ascribed to the osphradia, and it is dilficult 

 to understand the presence of two organs doing the same 

 work, situated very close to one another, and yet having 

 a different structure. 



The histological structure is remarkably like that of 

 the " Seitenorgane " described by Eisig (43) in the 

 Capitellidae, and Thiele himself supposes that the 

 function, like that of these organs and the lateral line of 

 fishes, is the perception of vibrations in the water, though 

 no experimental evidence is put forward. 



The Eye.s. — Xo organs of Pecten, nor of the Lamelli- 

 branchs in general, have given rise to more discussion 

 with regard to details of structure than the eyes : and as 

 one would expect, these remarkable organs, which are so 

 prominent, glittering like minute diamonds set amidst a 

 forest of tentacles, attracted the attention of early 

 naturalists. They were first described by Poli, as far 

 back as 1795. The first fundamental work was that by 

 Ilenseu in 1865 (31), and since then the most important 

 papers are those by Hickson in 1880 (33), Carriere 

 1885 (29), Patten 1886 (35), and Rawitz in 1888 (36). 

 The structure of the retina was, however, still very 

 uncertain, and this has been the chief feature of the 

 works of Schreiner 1896 (37), Ilesse 1900 and 1907 (32), 

 and Hyde 1903 (34), bringing the matter up to date. 

 The difficulty of making satisfactory preparations, and 

 the frequency of artifacts caused by fixation and further 

 treatment, has been the cause of much confusion. 



The mantle, as previously described, has a margin 

 bearing the characteristic lamellibranch structures (see 

 fig. 4). There are three folds, on the outer of which are 



