so-called Eyes of Tridacna. 439 



formations of the margin of the mantle, suitably selected por- 

 tions of the margin were stained with alum-carmine and then 

 divided in different directions into series of cross sections 

 from 0"0125 to 0"01 millim. in thickness. The result was 

 very surprising. From the scanty statements of Vaillant it 

 might have been anticipated that the histological investiga- 

 tion would reveal a highly organized eye. " Notwithstanding 

 the volume of these organs," says Vaillant (?. c. p. 135), 

 " which in large individuals measure not less than 2-3 millim. 

 at their base, I was unable, on account of the thickness and 

 opacity of the tissues, which render dissections very difficult, 

 in definitely finding in them the constituents which have 

 been described in some other MoUusca, and especially in the 

 species of Pecten. Near the apex of the tubercle there is a 

 spot of dark pigment, which may be regarded as a choroid ; 

 and, further, in successful preparations, when such a wart 

 is examined from the side, we see a convex transparent cap- 

 sule, which might perhaps be assimilated to a cornea." 

 These exceedingly cautious assertions, however, agree so 

 little with the actual conditions that it is difficult to say what 

 Vaillant really saw. For his excuse it may be indicated that 

 his method of investigation must necessarily have exposed 

 him to the most serious illusions. 



Let us first of all examine the larger wart-iike elevations 

 which stand at a distance from the margin of the mantle. In 

 these the first section in any direction teaches us the important 

 fact that the warts themselves are not eyes or other special 

 organs of any kind, inasmuch as their structure perfectly 

 agrees with that of the mantle. But in the warts themselves 

 there are in small numbers some very peculiarly constructed 

 organs of microscopic minuteness, which perhaps might be 

 eyes and which must here be somewhat closely examined in 

 the first place. 



The organs in question have the general form of a shallow 

 flask with a broad belly and a short wide neck. They lie 

 immediately beneath the epithelium and are so oriented that 

 the belly is turned inwards and the neck outwards, so that 

 their long axis is perpendicular to the epithelial surface. 

 Even on the largest warts we do not find more than ten or 

 twelve such organs ; on smaller ones fewer in proportion to 

 their size. With rare exceptions, in which we find individual 

 organs on the extreme periphery of a wart, they throughout 

 prefer the median regions ; their favourite place is the surface 

 of the wart turned towards the mantle-margin, which descends 

 abruptly towards the crescentiform furrow. Here we find 

 about 75 per cent, of their whole number, the remainder 



30* 



