444 M. J. Block on the Occurrence of 



A much more probable interpretation is that the flask-shaped 

 organs are luminous organs. If the cells of the " external 

 layer" have the faculty of shining, the " transparent cells " 

 might perhaps act as prisms. This depends, however, very 

 much on whether the distribution of the pigment, which we 

 do not know, supports such an interpretation. Whether any 

 luminosity really occurs during life is not known *, and, 

 indeed, not at all probable, as from the abundance of the Tri- 

 dacnce in the whole Indo-Pacific region so remarkable a phe- 

 nomenon could hardly have remained unobserved until now. 



Perhaps my respected friend Dr. Sluiter, when these lines 

 come under his notice, may be induced to make some obser- 

 vations upon the point in question. With the exception of 

 some superficial and illusory resemblances there is no relation 

 to the luminous organs of the Scopelidaa. The only organs 



closelyin structure with undoubted eyes (Patella, see P. Fraisse, Zeitschr. f. 

 -vviss. Zool. Bd. xxxv. p. 468 ; moreover, as 1 now find, Hilger has recently 

 demonstrated the nerve of the eye of Patella, see Morph. Jahrb. Bd. x. 

 p. 358, 1884), or at least because physiological experiments proved the 

 animal to be extraordinarily sensitive to light (as especially in the cases 

 recently described by Sharp and Patten ; see B. Sharp, " On the Visual 

 Organs in Lamellibranchiata," Mitth. Zool. Stat. Neapel, Bd. v. p. 447, 

 and W. Patten, " Eyes of Molluscs and Arthropods," ibid. Bd. vi. p. 542). 

 But no special sensitiveness to light can be absolutely proved in Tridacna ; 

 indeed, Tridacna is so little sensitive that usually it is only upon direct 

 contact that it retracts the margins of the mantle and closes its shell. 



• I certainly thought that I was on the track of a conclusive observa- 

 tion when I read as follows in the treatise on the Invertebrata, edited by 

 0. Schmidt, in Brehm's 'Thierleben ' (ed. 2, Bd. x. p. 387) :— " Besides 

 many singular things, as, for example, that the Giant Clams {Tridacna) 

 when they open at night diffuse a bright light or a lustre noticeable from 

 a distance . . . besides these things our Dutchman (Rumph) cites some 

 examples of the size and strength of Tridacna ffiffas," &c. But a com- 

 parison Avith the original showed that 0. Schmidt had either read the 

 passage in question hastily or misunderstood it. At p. 132 of his ' Am- 

 boinsche Kariteitkamer ' (first Amsterdam edition of 1705) Kumphius 

 saj's: — " They relate many singular things of a large Bia garu {Tridacna 

 gigas) whichis to be seen in a lagoon of the island Timor Laut, which on 

 opening at night is said to emit a bright light or lustre, which may even 

 be perceived from afar." Thus it is only a pleasant tale of the natives. 

 It may be remarked en passant that liumphius is well known to have 

 been a German, born in Hanau, as, indeed, is to be read upon the title- 

 page of his ' Kariteitkamer ' and also under his portrait behind it, although 

 " Totus Belga fide et calamo," as is added with an elegant compliment 

 to his adopted country in the distichs in his honour placed under the 

 latter. As, in my eyes, Rumph, although a dilettante, was a naturalist of 

 the first rank, who far exceeded most of his contemporary professional 

 naturalists not only in accuracy of observation, but also in critical acute- 

 ness, I would not let pass this opportunity of correcting an error which 

 may easily receive the widest diffusion through so popular a work as 

 Brehm's ' Thierleben.' 



