8 



PROTOZOA. 



of about 10 /a. It is succeeded by a series of chambers of gradually increasing 

 size, which are added one after another in a spiral manner. Numbers of small 

 nuclei which increase in number by simple division, are scattered apparently 

 irregularly through the protoplasm. The existence of the microspheric form, 

 as an individual, is terminated in the following manner. The animal becomes 

 firmly attached to some object by its psexidopodia, and the protoplasm emerges 

 from the shell. After involved streaming movements which continue for some 

 hours, the protoplasm divides simultaneously into a number of spherical masses, 

 having generally a diameter of about 70 /t., which, after secreting a shell, 

 rapidly draw apart from one another, and each sets up an independent existence. 

 These form the initial chambers of the 'megalospheric form of the species. The 



FIG. 5. mioculina depressa il'Orb. Sections of the shell : A, of the megalospheric form 

 (megalosphere 200-400 /*) longitudinal. li, of the microspheric form (microsphere 20 M-) 

 transverse. The two terminal chambers are omitted in II (from Lister, after Schlumberger). 



whole of the protoplasm of the parent is thus divided up to form the brood of 

 young, its empty shell falling to the bottom. The second chamber of the 

 megalospheric form of Polystomella has a peculiar shape, and the other 

 chambers added in succession build up the spiral shell. In this form a 

 single large nucleus is present, whose size increases witli the growth of the 

 protoplasm. Prior to the reproduction of the megalospheric form the large 

 nucleus disappears, and in place of it, though the way in which they are 

 produced lias not been followed, great numbers of minute nuclei are found 

 scattered through the protoplasm. After a preliminary division of these nuclei 

 by karyokinesis, the protoplasm breaks up into flagellated zoospores having a 



