THE MASTIGOPHORA 



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kinds have been distinguished, those that are imperforate (discoliths, 

 lopadoliths, calyptroliths), and those that have a central perforation 

 (Fig. 6, B) (placoliths and rhabdoliths). The significance of the per- 

 foration in the placoliths and rhabdoliths is not clear, but there is 

 no evidence at present that it transmits protoplasmic processes from 

 the ectoplasm. 



When the theca is once formed it is never increased in size by 

 the addition of new shells, but when the growth of the organism 



B 



G. 



FIG. 6. 



To illustrate the structure of the Coccolithophoridae. A, Scyphosphaera apsteini, Lohmann,. 

 X 2000. (j, a girdle of peculiar enlarged coccolitlis. B, optical vertical section of an example 

 of a perforated coccolith of Coccolithrrpora Irptopora, M. and B. C, side-view of a simple collar- 

 shaped imperforate coccolith of Calyptrosphaera oblonga, Lohmann. D, vertical section of 

 Pontosphuera haeckelii, Loh. ; co, the sheath of coccolitlis ; ch, the two chromatophores', eacli 

 containing a highly refractive globule ; /, the flagellum ; n, the nucleus. B, side-view of one 

 of the coccolitlis of the same species. F, Discosphaera tubifer, M. and B. ; ch, chromatophores. 

 G, trumpet -shaped projection from the coccolith of Discosphaera tubifer, x 2000. '(After 

 Lohmann and Murray and Blackman.) 



requires it, the theca is cast off as a whole and a new one formed 

 in its place. 



Reproduction is usually effected by simultaneous longitudinal 

 fission of the theca and protoplasm, but occasionally large thecae 

 are found containing two individuals, indicating that fission of the 

 protoplasm may precede division of the theca or the formation of 

 two thecae. 



No evidence has yet been obtained of the formation of gametes. 



The Coccolithophoridae are exclusively marine, but are found 

 everywhere except in pure polar waters. They reach their greatest 

 numbers at a few fathoms from the surface. 



