The Difflugia build their own shells, which are usually truncated spheres, ovate, or sometimes elongated into the form of a pitcher or flask. The most minute recognisable of these shells is about the 1/1000 of an inch in diameter, but they are constructed with the most perfect regularity. The Difflugia pyriformis or symmetrica has the form of an egg with an aperture at the small end. It is entirely made up of rectangular hyaline plates, arranged with the greatest regularity in consecutive transverse and longitudinal rows, the smaller ones being at the extremities, while the larger ones occupy the central and widest portion of the structure. The inhabitant of this abode is an Amœba with a sarcode body covered with a thin film, from whence it sends off pseudopodia 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 subsequent changes, and are replaced by a mass of colourless cells full of granules which are supposed to be the elements 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 circumstances. The greater number of the Difflugiæ secrete a substance 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 particles which it selects, and they adhere to it. The particles selected are invariably mineral matter. ‘The selective power is carried to such an extent that colourless particles—sometimes quartzose, sometimes felspathic, 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 vacant area; whilst in the specimens of Difflugia in which tabular or micaceous particles are used, they are sometimes disposed with such nicety that there is no overlapping, 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 Amœbæ in the strictest sense of the word; their bodies consist of sarcode, which sends out finger-like lobes from the mouth of the shell at one end, while the other end has an adhesive property, which fixes it to the bottom. The nucleus and contractile vesicles are identical in character with those of the Amœbæ, and exhibit the same tendency to subdivision at certain periods of the creature’s history that is witnessed on a large scale in the Amœba proper; and the reproductive process is the same.[5]