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protective shell secreted by the animal itself. The shell 

 is shaped like a watch-glass and made of chitin. It has a 

 hole (the pylome) in the centre through which the few 

 pseudopodia are protruded. A peculiar feature is the 

 presence of gas vacuoles which give a certain buoyancy 

 and probably help to balance the shell. There are two 

 distinct nuclei and numerous nuclear particles. 



Reproduction is by fission (the protoplasm streams out, 

 another shell is formed, and then division takes place), 

 or by spore formation in which there is conjugation of the 

 gametes. 



DIFFLUGIA is common in ponds creeping on the surface 

 of the mud or amongst the tangled masses of Confervae. 

 It forms a pear-shaped skell of grains of sand glued together 

 with a secretion of the protoplasm. It has several nuclei ; 

 otherwise, in its structure and mode of reproduction, it 

 resembles Arcella. 



The shell and the consequent restriction of pseudopodia, 

 the number of nuclei, and the gas vacuoles in Arcella, are 

 the chief differences between these forms and Amoeba. 



CLASS HELIOZOA (SUN-ANIMALCULES). 



What are the General Features of the Heliozoa ? 



The body is spheroidal and only to a slight extent shows 

 amoeboid change of shape. The protoplasm is full of non- 

 contractile vacuoles, more especially the outer portion 

 (ectoplasm), and this may be associated with the habit 

 of floating passively in the water. 



The numerous pseudopodia are long, slender, and project 

 stiffly like radii from the body. Each consists of a thin 

 layer of protoplasm around an elastic axial filament, the 

 base of which is (in Actinophrys, but not in Aclinosphcerium) 

 in contact with the nuclear membrane. There may be 

 only one nucleus (e.g., Actinophrys) or many (e.g., Actino- 

 sphcerium which has at least 200). Besides the non-con- 

 tractile vacuoles mentioned, there are food vacuoles and 



