86 



PROTOZOA THE SIMPLEST ANIMALS. 



parts of the food have been absorbed, the undigested residue 

 is got rid of at any point of the protoplasm. One or more 

 contractile vacuoles are visible in the cell-substance. They 

 have an excretory function, and serve to get rid of the finer 

 waste products. 



Life history. — In favourable nutritive conditions the 

 Amceba grows. At the limit of growth it reproduces by 

 dividing into two. In disadvantageous conditions, such as 

 drought, it may become globular, and, secreting a cell-wall 

 or cyst, lie dormant for a time. The cyst-wall is said to be 

 chitinoid. With the return of favourable conditions the 

 Amceba revives, and, bursting from the cyst with renewed 



Fig. 33. — Life history of Amceba. 



l. Amceba with pseudopodia ; «., nucleus; c.v., contractile vacuole. 2. Division 

 in two. 3. Encystation. 4. Escape of Amceba from its cyst. 



energy recommences the cell-cycle. The conjugation of 

 two Amoebae has been observed, and spore formation oc- 

 casionally occurs. 



Second Type — Gregarina. 



Gregarina, a type of those Sporozoa in which the cell is 

 divided into two regions by a partition. 



Description. — Various species occur in the intestine of 

 the lobster, cockroach, and other Arthropods. When young 

 they are intracellular parasites, but later they become free in 

 the gut. They feed by absorbing diffusible food stuffs, such 

 as peptones and carbohydrates, from their hosts, and store 

 up glycogen within themselves. In many the size is 

 about one-tenth of an inch. There is a firm cuticle of 

 " protoelastin," which grows inwards so as to divide the 



