ANIMAL NATURE OF DIATOMEJB. 363 



for instance, it is always by the lateral surfaces that they 

 touch each other; and since all other characters some- 

 times fail, we can affix to them the denomination lateral 

 from this principal one. 



Besides the* vittap, before mentioned, in some genera 

 (Biddulp/tia, Clemacosphenia, Terpsinoe) there are other 

 solid substances in their internal cavities ; these are 

 variously arranged. 



These essential peculiarities of the shield may perhaps 

 .be regarded as indicating a complex structure, very 

 different, therefore, from what would be prescribed by a 

 simple cellular wall. Ehrenberg deduces from it an 

 argument to compare it with the shell of Mollusca. 

 The Arcellina3 may be cited among the Infusoria. 

 Kiitzing states, in reply, that among vegetable cells 

 there is found a peculiar conformation of the walls, with 

 prominences, depressions, points, lines, papilla, and 

 perforations, disposed in a regular manner; he refers 

 to grains of pollen, as an instance. He might have 

 added the more appropriate instance of the Desmidieas, 

 which would be very closely allied to the Diatomeas, 

 if the latter, like the former, could be referred to the 

 vegetable kingdom. If not equal in constancy and 

 regularity, the Desmideas display a greater degree of 

 complication; and we must remember the different 

 nature of their substance, for in the vegetable cell, when 

 lime or silica predominates, the wall becomes uniform and 

 regular (?) (in the text uniforme ed z'rregolare.) 



The soft internal substance is brownish yellow, or of 

 a gold colour. It is so described by Kiitzing. At first 

 it is almost always homogeneous ; at a later period it be- 

 comes granular, and divides into lobes, or contracts into 

 many spherical bodies, or only into one larger globule. 

 The development, forms, and distribution vary in different 

 groups, but are characteristic in the greater number of 

 these. Usually at first there is formed a continuous 

 membrane, which afterwards splits longitudinally, and at 



