SECT. ii. ARCELLA AND D1FFLUGIA. 23 



covered with a thin film, from whence it sends off pseu- 

 dopodia through the mouth of its shell. The Difflugia 

 is propagated by conjugation, but before that takes 

 place it becomes densely charged with chlorophyll-cells 

 and starch-grains. The former disappear during the sub- 

 sequent changes, and are replaced by a mass of colourless 

 cells full of granules which are supposed to be the ele- 

 ments of a new generation. The embryo or earliest 

 form is a minute truncated sphere, but the animal builds 

 up its habitation very much according to local circum- 

 stances. 



The greater number of the Difflugise secrete a sub- 

 stance which forms a smooth layer in the interior, 

 which the animal covers with sarcode from its mouth, 

 and then it drags itself with its pseudopodia to the par- 

 ticles which it selects, and they adhere to it. The par- 

 ticles selected are invariably mineral matter. 'The 

 selective power is carried to such an extent that colour- 

 less particles sometimes quartzose, sometimes felspa- 

 thic, sometimes micaceous are always chosen.' 'The 

 particles seem to be impacted into the soft matter, laid 

 on the exterior in the same way that a brick is pressed 

 into the yielding mortar, and that too, in so skilful a 

 manner as to leave the smallest possible amount of va- 

 cant area ; whilst in the specimens of Difflugia in which 

 tabular or micaceous particles are used, they are some- 

 times disposed with such nicety that there is no over- 

 lapping, but the small fragments are placed so as to 

 occupy the space left between the larger ones. These 

 excellent architects seem to know that in the valves of 

 the Diatoms are combined the properties best suited to 

 their wants, that is, transparency and form, capable of 

 being easily arranged.' 



Both the Difflugia and Arcella are Amoebae in the 

 strictest sense of the word; their bodies consist of 

 sarcode, which sends out finger-like lobes from the 



