158 THE SEA. 



the microscope, will be to take a low pill-box of card, 

 and suppose it to be made of flint glass, delicately 

 sculptured, and reduced to an invisible minuteness. 

 Suppose the granules of the yellow endochrome to be 

 enclosed in this box, surrounding a central mass called 

 the neucleus, which seems to be the very heart, or soul, 

 or life-point of the tiny organism. The cover and 

 bottom disks, caUed valves, are very easily separable 

 from the hoop that unites them (a parallelism that too 

 often obtains in their cardboard representative;) and 

 so these single valves are often found alone. 



The name Diatoma, which, originally given to a 

 single genus, has been applied to the whole order, has 

 reference to the readiness with which the strings or 

 chains in which most of the forms are aggregated, 

 may be separated, looking as if divided by a sharp 

 cut, either partially or wholly. And this depends on 

 their mode of multiplication. For the law by which 

 these atoms increase is highly curious. The pill-box- 

 like frustule becomes deeper by the widening of the 

 hoop, thus pushing the valves further from each 

 other ; then across the middle two membranes form, 

 which, by-and-by, by the deposition of flinty matter, 

 become glassy valves, corresponding to the two outer 

 valves ; and then the whole frustule separates between 

 these two new valves, and form two frustules. The 

 old hoop (in some cases at least) falls off, or allows the 

 hoops of the new-made frustules to slip out of it, like 

 the inner tubes out of a telescope. 



Now the separation, of the frustules thus made is 



