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chromidia (scattered granules of nuclear chromatin), 

 whereas the microspheric has a number of small nuclei 

 scattered throughout. Dimorphism of individuals is 

 therefore characteristic of Polystomella. 



It is an abundant littoral form, found upon Zostera and 

 seaweeds^(e.g., Laminaria). 



Describe the Life-history of Polystomella. 



When the microspheric form is 'about to reproduce, 

 there is a multiplication of nuclei by division, and the 

 pseudopodia become so very numerous that they form 

 a sort of halo around the shell. The entire body then comes 

 out of the shell, and (in the halo) breaks up (by fission) 

 into a number of small round pieces. Each secretes a tiny 

 shell with one aperture; it soon forms a second chamber, 

 and is then seen to be a two-chambered young megalo- 

 spheric individual. When it is fully grown and about to 

 reproduce, the nucleus disappears and is replaced by a 

 number of minute nuclei. Then the body breaks up into 

 rounded portions each with a nucleus. These embodied 

 nuclei now divide by karyokinesis ; and this is followed 

 by a further division into rounded bodies each containing 

 one of the daughter nuclei. That is the process of sporu- 

 lation. 



The hosts of little units have flagella, and they issue 

 (as gametes or zoospores) from the shell (now empty). 

 They conjugate in pairs (a gamete of one brood with one 

 from a different brood) to form zygotes ; and from these 

 zygotes the microspheric forms are developed. This life- 

 cycle illustrates alternation of generations. 



Point out the Special Interest of (a) Globigerina, (b) Hali- 



physema. 



GLOBIGERINA has much more protoplasm outside the 

 shell than inside it, and the outer mass is *full of vacuoles, 

 enabling the animal to float at the surface of the sea. 

 Amongst the outspread net-like pseudopodia there are 

 long and delicate, hollow limy spines (set on the shell) 

 which have an axis of protoplasm. The important chalky 



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