i PROTOZOA 19 



secreted material, somewhat hornlike in appearance, shaped like a 

 concavo-convex lens, and possessing a circular opening in the middle of 

 the concave side. In the case of Difflugia the shell or house is on the 

 other hand built up of foreign particles, it may be small grains of sand, 

 or particles of the skeletons of freshwater animals, fitted closely together 

 over its surface and held together by a cement secreted by the animal's 

 protoplasm. An important physiological peculiarity of Arcella is that 

 it habitually produces in its protoplasm gas-vacuoles 1 which counteract 

 the weight of the shell. It is said that when the Arcella is capsized on 

 to its convex side it is able to right itself by active formation of gas 

 vacuoles which buoy it up and enable it to recover its normal 

 position. 



POLYSTOMELLA 



Of the FORAMINIFERA it will be convenient to study in the first instance 

 a special example namely Polystomella which can be easily investigated 

 in the laboratory. If handfuls of seaweed are plucked from rocks near 

 low-water mark and washed to and fro in a dish of clean sea-water it 

 will be found that numerous grains of " sand " are washed out of the 

 seaweed and sink to the bottom of the vessel. If now the vessel be left 

 undisturbed it will be found after a few hours that some of the apparent 

 grains of sand have crept up the sides of the vessel and are attached to it. 

 They are really living Foraminifera and amongst them one can often 

 recognize Polystomella by the characteristic brown colour of its cytoplasm. 

 Examined with a low power of the microscope the Polystomella is seen 

 (Fig. 7) to have its body coiled in a flat spiral. It is supported by a 

 shell or skeleton composed mainly of calcium carbonate, and divided up 

 into successive chambers which increase in size from the centre outwards 

 each chamber fitting spoonwise ' over the chamber on its inner side. 

 Each chamber is filled by a mass of protoplasm the form of which is 

 readily seen on dissolving away the opaque calcium carbonate by means 

 of acid. A characteristic detail which enables one to distinguish Poly- 

 stomella from other and somewhat similar Foraminifera is that the edge 

 of each protoplasmic mass, except those nearest the centre, projects 

 backwards in a row of peglike extensions over its predecessor (Fig. 8, 

 i and 7, p). The successive masses of protoplasm are not completely 

 isolated from one another : each mass is continuous with its two neigh- 

 bours by one or more bridges of protoplasm towards its central end. The 

 outer wall of each chamber is perforated by numerous minute pores or 



1 The gas has generally been regarded as carbon dioxide but more recent investi- 

 gations (Bles) suggest that it is oxygen. 



C i 



