20 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



foramina and through these the protoplasm is continued outwards to 

 form a thin layer of external protoplasm which ensheathes the whole" shell. 

 The shell is therefore not an external shell like that of a snail but an 

 internal skeleton completely embedded in living substance. The external 

 protoplasm extends outwards when the creature is active into pseudo- 



FlG. /. 



Polystomella, alive. Certain of the pseudopodia towards the lower side of the figure are flowing 

 round a particle of food. 



podia (Fig. 7) which are however in appearance totally unlike those of 

 Amoeba. They are very long slender threads of protoplasm which show 

 a marked tendency to fuse in places with their neighbours. With a 

 hi-h magnification it may be seen that there is a constant slow circulation 

 in the extruded pseudopodium the cytoplasm streaming outwards along 



