20 PROTOZOA. FORAMINIFERA 



commonly, but not always, grows to a larger size than in the 

 megalospheric type, and individuals of the former are 

 much less numerous than of the latter ; in other respects 

 the two are similar. The relationship of the microspheric 

 and megalospheric shells has been elucidated by a study of 

 the life-history of Polystomella and other living Forami- 

 nifera. When reproduction takes place in the microspheric 

 form all the protoplasm passes out of the shell and divides 

 into spherical masses, each of which secretes a shell and 

 develops into a megalospheric individual. In the repro- 

 duction of the megalospheric form the protoplasm divides 

 into small rounded portions which pass out of the shell as 

 moving spores — zoospores ; it is believed that two zoospores 

 from different individuals conjugate and give rise to a 

 microspheric individual. There are, therefore, two modes 

 of reproduction — asexual and sexual, which alternate. 



For convenience of reference the Foraminifera may be 

 divided into three groups, the characters of which are 

 based on the structure and composition of the shell ; but 

 this cannot be regarded as a natural classification since in 

 some types which are usually calcareous {e.g. Miliola) we 

 occasionally meet with species in which the shell consists 

 largely of sandy material. 



I. Porcellanous Forms. 



Shell calcareous, porcellanous, not perforated by canals, 

 but provided with one or two large apertures through which 

 the pseudopodia pass out. 



Miliola. (fig. 3, A — D.) Shell multilocular, the chambers 

 being coiled on an elongated axis, each chamber forming half a 

 convolution. In some cases all the chambers are visible externally 

 on both sides of the shell (fig. 3, D) ; in others, owing to the 

 lateral prolongations of the chambers, only the last one or two are 



