70 



EVENINGS AT THE MICROSCOPE. 



Avill just point out a beautiful though minute proof of 

 design in a point of the structure of the cells connected 

 with these pearly chambers. If von look closely, you 

 ■will see that the spines of the margin are not found on 

 those cells that carry the pearls ; and moreover, that 

 they are also -svanting on the approximate edges of the 

 two cells that lie behind every such pearl-bearing cell. 

 Now the reason of this omission is obvious. The spines 



LEArY SEA-MAT. 



(A portion magnified.) 



projecting obliquely would interfere with the gaping of 

 the door; and hence they are invariably absent there. 



I happen to have in my aquarium a living individ- 

 nal of another species belonging to the same class, and 

 agreeing with this in all essential particulars of struc- 

 ture, though widely different in form. The difference, 

 however, is mainly dependent on a rather miiniportant 

 point of arrangement ; for the cells, instead of being set 

 side by side and end to end in quincunx fjishion, to an 

 indefinite extent, on two surfaces of a plane, are dis- 

 }">osed on one single surface, and in longitudinal rows 



