62 



THE FORAMINIFERA 



they might with more reason be regarded as representing the two 

 sexes of a species. The authors did not, however, abandon the 

 old idea of the specific distinctness of the two forms. 



Investigation of other genera of Foraminifera has shown that 

 the phenomenon of dimorphism is, as Munier-Chalmas expected, 

 widely found among them. 



The relation between the two forms will be best elucidated by 

 examining the structure and life-history of a single species. For 

 this purpose we will select Polystomella crispa (L.), the life-history 

 of which is most completely known. 



THE STRUCTURE OF POLYSTOMELLA CRISPA. This is one of 

 the most abundant of the littoral Foraminifera. It lives in shore 

 pools and down to a depth of 355 fathoms, and ranges from Green- 

 land in the north and Kerguelen Island in the south to the equator. 

 It is very common on our own shores. 



The test is biconvex and symmetrical about the median plane. 

 The chambers are arranged in a spiral series, and are equitant, i.e. 

 they bestride the chambers of the preceding convolution and over- 

 lap them at the sides, each being prolonged in what are known as 

 alar prolongations, which extend towards the spiral axis of the 

 test. Partly as the result of this overlapping, and partly because 

 the axial region is filled in with canalicular shell substance, only 

 the last convolution of chambers is visible externally. 



On the terminal face of the last chamber, where this face 



joins the wall of the 

 preceding convolution, a 

 V-shaped line of pores 

 (Fig. 7, a) is visible. 

 These represent the aper- 

 ture of the test, and are 

 the main channels of 

 communication between 

 the terminal chamber 

 and the exterior. At the 

 posterior margin (i.e. the 

 margin remote from the 

 terminal face) of each 

 chamber a number of 

 pocket -like prominences, 

 the retral processes (Figs. 

 7 and 8, r), project back- 

 wards, and are marked by 

 ridges on the external 



surface. They end blindly, but are separated by pits, at the 

 bottoms of which are the openings of branches of the canal 

 system, which will be described later. The outer surface of 



Fio. 7. 



X about 40. 

 , retral pro- 



The test of Pdystvmtlla crispa (L.> 

 a, the line of terminal apertures ; 

 cesses ; between them are seen the pits by which 

 branches of the canal system communicate with the 

 exterior. 



