442 M. J. Brock on the 



them. Here also we find flask-shaped organs in the typical 

 position at the inner wall of the " fosse," or more rarely upon 

 the still very low dome of the wart which is just swelling up. 

 But their number is small, varying only from one to three ; 

 and moreover, as already stated, they project more strongly 

 above the surface, and push out tlie much thinned epithelium 

 of the mantle into a bump at this spot. 



With respect to size and the details of the minute structure 

 there is, however, absolutely no difference between them and 

 the flask-shaped organs of the larger warts. If we examine 

 more exactly the external relief of these younger structures it 

 is not difficult to find evident incipient stages, in which only 

 a deep narrow inversion of the epithelium of the mantle 

 represents the first trace of the future " fosse," while within 

 this the future tubercle is either not indicated at all or only 

 by a broad, low, scarcely perceptible elevation. From these 

 incipient stages up to typically developed warts all possible 

 intermediate forms may be found, and hence there seems to 

 us to be no doubt that the smaller structures near the margin 

 of the mantle are developmental stages of the typical large 

 warts. If this conclusion be correct, this development has 

 certainly the remarkable peculiarity of showing that first of all 

 the " fosse " surrounding the wart on the side towards the 

 mantle-margin sinks in, and the wart only then begins to 

 swell up above the surface of the mantle. If we add to this 

 that our younger specimen of Tridacna, in which the mantle- 

 margin measures only 13 centim. in length, shows only such 

 younger organs towards the margin, and, indeed, in rather small 

 numbers (in all about fifty were counted upon each mantle- 

 margin), and, on the other hand, not a single distinctly pro- 

 jecting wart, we are not unjustified in coming to the conclu- 

 sion that during the whole life of the animal new-formation 

 of warts goes on continually, starting from the margin of the 

 mantle *. 



With regard to the formation of the flask-shaped organs 

 we have unfortunately no direct observation. But from the 

 circumstance that they occur typically developed in the 

 youngest warts, it at least follows with some certainty that 

 their formation precedes that of the warts. Therefore as the 

 place where a new flask-shaped organ originates is not marked 

 externally by change in relief of the surface of any kind, it 

 is clear that only some very fortunate chance could throw any 

 light upon the production of a flask-shaped organ in the adult 



* The deTelopment of the warts sometimes attains such an extreme 

 degree that they begin to be constricted off from tlie parent-surface and 

 give origin to sliort-stalked clavate or mushroom-like structures. 



