SEA-URCHINS AND SEA-CUCUMBERS 317 



that when we are examining a section thin enough to con- 

 tain two or three such layers, it is easy, by properly 'fo- 

 cusing' the microscope, to bring either one of them into 

 distinct view. From this very simple but very beautiful 

 arrangement, it comes to pass that the plates of which the 

 entire 'test' is made up possess a very considerable degree 

 of strength, notwithstanding that their porousness is such 

 that if a portion of a fractured edge, or any other part from 

 which the investing membrane has been removed, be laid 

 upon fluid of almost any description, this will be rapidly 

 sucked up into its substance." ' 



To return, however, to our spine. When we look at it 

 laterally, the appearance is such that we cannot but firmly 

 believe that it is grooved throughout with straight and 

 deep longitudinal furrows. But if we break off the same 

 spine transversely, and so exhibit it that the broken end 

 shall be presented to the eye, we perceive that there are 

 no grooves; but that the points in the circumference, 

 which seemed to be the summits of the ridges, which are 

 very narrow, are really lower than the intermediate spaces, 

 which we supposed to be the grooves, and that the surface 

 of these spaces is really convex in a slight degree. 



The explanation of these contradictory appearances is 

 easily given. Meanwhile, however, they read an impor- 

 tant lesson to the inexperienced microscopist, not to decide 

 too hastily on the character of a surface or a structure, 

 from one aspect merely. So many are the chances of illu- 

 sion, that the student should always seek to view his sub- 

 ject in different aspects, and under varying conditions of 

 light, position, etc. 



1 "The Microscope," p. 553. 



