436 M. J. Brock 07i the 



beauty. They stand out from the margin of the mantle, 

 which is sometimes ultramarine blue, sometimes emerald- 

 green *, as an irregular row of differently coloured points, 

 sometimes black, sometimes brown f, so that an impi'ession 

 is produced as if Nature, in order to heighten the brilliant 

 spectacle, had set differently coloured gems in the splendid 

 material of which she forms the margins of the mantle. 

 Even upon a superficial examination we easily see that these 

 differently coloured spots adorn the summits of low, obtusely 

 conical elevations, which Vaillant directly characterizes as 

 "eye-tentacles" (" tentacules oculiformes," I. c. p. 83). 

 How far this is correct a closer examination of their structure 

 will show. 



The considerable size which, as is well known, these animals 

 attain, and the labour necessary for obtaining them (they 

 have to be chiselled out of tlie blocks of madrepore J), at once 

 placed a limit upon the amount of material brought away. 

 My investigations have therefore been made exclusively upon 

 three specimens ; but as these furnished me with several 

 hundred " eyes " for examination, the want of very young 

 and of full-grown examples can alone be regarded as inju- 

 rious to the completeness of the description. My largest spe- 

 cimen, which measured 18 centim. along the margin of the 

 mantle, was killed in very dilute chromic acid (0'25 per cent.), 

 then treated with gradually stronger alcohol ; in the case of 

 a second specimen of the same size the margin of the mantle 

 was separated from the living animal and hardened succes- 

 sively in dilute osmic solution and then in alcohol ; with a 

 third small example I contented myself with hardening in 

 alcohol. As will be seen hereafter these different methods of 



* In Tridacna crocea, Lam., according to Quoy and Gaimard, ultra- 

 marine blue, in T. elonyata, Lam., green, in T. squamosa, Lam., wliicli 

 was observed by me, most frequently also green, but with all shades 

 towards blue very frequent, until the animals were pure blue. Moreover 

 the metallic lustre of the colours is so strong that, as Vaillant correctly 

 remarks {I. c. p. 73) only comparison with jewels can give even a tolerably 

 good notion of them. The pigment, according to Vaillant (/. c. p. 86) is 

 exclusively seated in the epithelium of the mantle. At any rate the 

 coloration disappears immediately in alcohol without leaving any traces ; 

 it must also be remarked that nothing is to be foimd of a " spangle- 

 layer," such as is so generally diffused in fishes with metallic lustre. 



t Black in T. elonyatu, observed by Vaillant (which is confirmed by 

 Mobius, ' Beitrage zur Meeresfauna d. Insel Mauritius u. d. Seychellen, 

 Berlin, ]880, p. 322), yellowish green in T. crocea according to Quoy and 

 Gaimard, as also from my personal recollections in T. squamosa. Unfor- 

 tunately I cannot now make any definite statement upon this point, as I 

 omitted making a coloured drawing. 



X The mode of life of T. squamosa appears therefore to agree perfectly 

 with that of T. crocea, as described by Quoy and Gaimard {I. e. p. 488). 



