Lang DON, Se7ise-organs of Nereis vireiis. 59 



above described takes place and thus being still able to change 

 their form under the influence of surface tension and also of 

 external forces. 



When the upper halves of the peripheral processes became 

 pulled away from the tube, they would also be separated from 

 the lower halves of the processes next above them and this line 

 of separation would be a line passing spirally arou?id the ceyitral 

 tube. The truth of this statement can be easily demonstrated. 

 If a surface be covered with hexagons, it will be at once seen 

 that the line of least resistance — the line along which these hex- 

 agons would most easily separate — is an oblique one. If the 

 surface of a thin plate of clay be marked with hexagons and 

 the plate carefully invaginated so that the hexagons are on the 

 outer surface of the invagination, and then a line be traced 

 through the hexagons along the line of least resistance, this line 

 will pass spirally around tJie invagination and if the hexagons are 

 very small, it will need but one such spiral to take in every hexagon 

 on the invagination. This shows that in an invaginated surface 

 formed of small hexagonal cells, the line along which these cells 

 will tend to break apart under strain is a spiral one winding 

 around the invagination. This, it seems to me, accounts for 

 the spiral arrangement, characteristic of the spiral organ. 



The above explanation needs to be tested by a study of 

 the development of these organs. It is, however, an explana- 

 tion built upon general facts of development, for it will be con- 

 ceded that ocular organs come from simple epidermal areas 

 which often become invaginated, that epidermis increases in 

 height during development, and that the bodies of epidermal 

 cells, especially nerve-cells, often wander from their original po- 

 sitions. The truth of the mechanical principle involved can 

 easily be tested. In ocular organs of other forms in which the 

 refractive bodies are massed together, the cell bodies lie close 

 to the refractive bodies and there has been no change of position 

 which would cause the refractive bodies to separate from one 

 another. 



The position of these spiral organs on the dorsal and lat- 

 eral surfaces of Nereis and in the tips of the gill-lobes is that 



