II] 



ARCELLA 



21 



addition a cloud of minute nuclear particles known collectively as 

 a chromidium. The cliromidium consists of material emitted by 

 the nucleus of the germ before it divides into the two nuclei seen in 

 the adult. When Arcella reproduces a large part of the protoplasm 

 emerges from the shell, each primary nucleus then divides into two, 

 and two of the daughter nuclei take up their positions in the outside 

 mass which is then cut off as a new Arcella. At other times the 

 primary nuclei dissolve and disappear, and the protoplasm aggregates 



FIG. 3. Arcella discoides x 500. From Leidy. 



A. Seen from above. B. Seen from the side, optical section. 1. Shell. 

 2. Pseudopodia. 3. Edge of opening into shell. 4. Thread attaching 

 animal to inner surface of shell. 5. Nucleus. 6. Food vacuole. 



7. Gas vacuole. 



itself as spherical lumps round the minute particles of the chro- 

 midium which form secondary nuclei. These ball-like masses are 

 spores ; they are extruded one by one from the shell and at first they 

 put out stiff pseudopodia like Actinophrys (see below). These minute 

 germs unite in pairs and the compound organism is called a zygote 

 and grows up into a young Arcella. The union of two germs so as to 

 form a single germ which grows vigorously is the simplest form of the 



