so-called Eyes of Tridacna. 443 



animal. It may, however, be admitted that for a long time 

 a new- formation of such organs must take place in the larger 

 warts, for a comparison of the number of flask-shaped organs 

 of the larger warts with the very much smaller immber 

 in younger structures leaves only the alternative between 

 this supposition and the much more improbable one of a sub- 

 sequent fusion of several smaller warts into one large one ; 

 but even here it is remarkable that I have never been able 

 to observe a flask-shaped organ {71 statu nascendi. How- 

 ever, I will readily admit that I have not devoted much time 

 to seeking for it, as in connexion with the chief interest which 

 attaches to these mysterious organs, namely their function, 

 no light is to be expected from developmental history. 



Throughout, in all attempts to attribute any definite func- 

 tion to these organs, we find ourselves in a peculiarly unfa- 

 vourable position. In the first place, because there are no 

 available observations as to the behaviour of the living 

 animal, as to undoubted sensorial perceptions, or the like, 

 which might be brought into connexion with the flask-shaped 

 organs. Further, because we know nothing definite as to 

 the pigment, which, as we have seen, is undoubtedly present 

 during life, and its arrangement relatively to the organs, a 

 point which would have to be considered in the first line in 

 every attempt at interpretation. And, finally, because we 

 know even the histological elements which compose the 

 flask-shaped organs only in the preserved state. We do not 

 know whether the transparent cells are as strongly refractive 

 during life as we suppose them to be from our preparations ; 

 we know nothing as to the constitution of the cells of the 

 " external layer" during life. Thus any serious attempt at 

 interpretation must for the present remain in suspense. 

 Nevertheless we believe we shall not go wrong in decidedly 

 rejecting any interpretation of our organs as eyes. The only 

 things that might be adduced in favour of this interpretation 

 is the resemblance of the " transparent cells " to the lenses of 

 many Invertebrates, and above all the exceedingly favourable 

 position of the organs for visual perception ; but how many 

 and important are the reasons against it ! Above all, at any 

 rate, tlie want of any large nerve-trunk running to the organ 

 and of a perceptive layer, for we cannot expect even the 

 boldest imagination to regard, the " external layer " as a 

 retina *. 



* We know very well that precisely among the Mollusca "eyes" have 

 frequently been desciibed to which no large nerve-trunks could be proved 

 to run ; but in all these cases the interpretation, alihough assailable, is to 

 a certain extent justified, if only because the organs in question agreed 



