SEA-UECHrNS AND SEA-CUCUMBEES. 321 



to the other in a direction perpendicular to their plane, 

 are so arranged that the perforations in one shall cor- 

 respond to the intermediate solid structure in the next, 

 and their transparency is such, that when we are ex- 

 amining a section thin enough to contain two or three 

 such layers, it is easy, by properly 'focussing' the 

 microscope, to bring either one of them into distinct 

 view. From this very simple but very beautiful ar- 

 rangement, it comes to pass that the plates of which 

 the entire ' test ' is made up, possess a very considera- 

 able 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. Eut 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 

 important lesson to the inexperienced microscopist, 

 not to decide too hastily on the character of a surface 



* The Microscope, p. 653. 

 14* 



