32 



condition is often a phase in the life-history of one species. 

 Contractile vacuoles are absent in the marine forms. 



In several instances two distinct kinds of individuals 

 occur (dimorphism) in the course of n lite cycle (e.g., 

 Polystomella}. Some " Forams " occur in moor-pools, 

 but most are marine. Most of the marine forms frequent 

 the shallow inshore waters (e.g., Polystomella} ; but some 

 are found in the deep sea, and many are pelagic (e.g., 

 Globigerina). 



Write a short Description of the Structure of Polystomella^ 

 and state where it is found. 



The shell is biconvex. Its chambers are formed in a 

 close spiral ; each convolution bestrides the one preceding 

 and overlaps it at the sides, so that only the last whorl 

 is seen from the outside. A V-shaped row of holes on the 

 end of the last chamber represents the principal aperture. 

 The pores of the chamber walls are very minute. 



The shell is covered by a thin outer layer of protoplasm, 

 and from it the pseudopodia are given off, some also issu- 

 ing from the V-shaped pores. The viscid pseudopodia are 

 slender and thread-like, some are grouped together like 

 sheaves, others are fused or interlaced at places. 



When a number of specimens (apparently similar) are 

 placed in acidulated solution of corrosive sublimate, the 

 shell is dissolved and the protoplasmic body is preserved. 

 The body is in portions, corresponding to the chain bers. 

 These portions are united by little bridges of protoplasm 

 and each has a posterior row of pocket-like or retral pro- 

 cesses. But the specimens are not all the same. Most of 

 them have a large central and initial chamber (i.e., megalo- 

 spheric) ; but, perhaps, one (one in about thirty) is found 

 which has a very small central chamber (i.e., microspheric). 

 In the megalospheric type the retral processes are present 

 on all the portions except the megalosphere ; but in 1 he 

 microspheric type they are absent from the portions of 

 the first convolution. Further, when the specimens ;ne 

 stained (to show the nuclei), it is then seen that the megalo- 

 spheric form has only one large nucleus and numerous 



