86 THE LOBOSA 



the observation of it in the living animal, but it seems probable that 

 one or more contractile vacuoles are present in all genera. 



Nucleus. For a considerable period in the life-history of Arcella 

 there are two large oval nuclei, from '01 5-0*0 2 mm. in diameter, 

 which are usually situated some distance apart, near the periphery 

 of the cytoplasm. More rarely three or even four of these 

 relatively large nuclei may be found. These nuclei are derived by 

 the karyokinetic division of the primary single nucleus of the young 

 Arcella. Each nucleus contains a single large ('008 mm.) nucleolus, 

 which apparently consists mainly of chromatin, but is otherwise 

 clear and transparent (Fig. 21). 



In other Thecamoebida (Difflugia 1 and Centropyxis) there is 

 usually only one nucleus during the corresponding phase of the life- 

 history, and this exhibits a coarse reticulum of chromatin with 

 numerous nucleoli distributed through it. 



The chromidial network of Arcella is in the form of an 

 irregular band or ring at the periphery of the cytoplasm, which 

 sends lobate processes or branches in the direction of the central 

 protoplasm. These processes are sometimes pinched off from the 

 peripheral ring, and appear as isolated patches of the chromidial 

 network in the central cytoplasm. 



In Centropyxis the chromidial network is in the form of a thick 

 sickle-shaped band lying in contact with the convex aboral 

 extremity of the body. Sometimes this band envelops the 

 nucleus, but neither in Centropyxis nor in Arcella does the nucleus 

 come into contact with the network, being always surrounded by a 

 halo of clear protoplasm (Fig. 21). In some forms of Difflugia 

 the chromidial network is in contact with the nucleus (Fig. 6) ; 

 in D. globosa and others, however, there is a clear space between the 

 nucleus and the chromidial network as in Centropyxis, but in 

 these cases strands of the chromatin seem to connect the nucleus 

 with the network. 



In another phase of the life -history of Arcella there are 

 numerous nuclei. The number is very variable, from 5 to 39, but 

 in a great many cases there are about 25. These secondary nuclei 

 are formed by the concentration of granules of chromatin of the 

 chromidial network, which become rounded off and surrounded by 

 a nuclear membrane. The larger the number of nuclei, the smaller 

 they are. When very numerous these nuclei are not more than 

 0'009-0'01 mm. in diameter. As the secondary nuclei are formed, 

 the two or three primary nuclei degenerate and disappear. 



When a certain number of secondary nuclei have been formed, 



they divide by karyokinesis. This karyokinesis is a preparation for 



the process of fission. One half of the nuclei resulting from the 



karyokinetic division remain at the periphery, the remaining half 



1 According to Zuelzer there are 10-30 nuclei in D. urceolata, Carter. 



