THE MASTIGOPHORA 



175 



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 



Fro. 6. 



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

 x 2000. g, a girdle of peculiar enlarged coccoliths. B, optical vertical section of an example 

 of a perforated coccolith of Coccolithopora Icptopora, M. and B. C, side-view of a simple collar- 

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

 Pontosphaera haeckelii, Loh. ; co, the sheath of coccoliths ; eh, the two chromatophores, each 

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

 of the coccoliths 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. 



