104 THE SEA SHORE 



then the softer protoplasm contracts in the middle, and finally 

 divides into two portions, each of which contains one of the nuclei. 

 The two distinct animals thus produced both grow until they 

 reach the dimensions of their common progenitor. 



All the protozoons resemble the 

 amoeba in general structure and 

 function ; but while some are even 

 simpler in organisation, others are 

 more highly specialised. Some, 

 like the amoeba, are unicellular 

 FIG. 53. THE AM<EBA, DIVIDING animals ; that is, they consist of a 



single, simple speck of protoplasm ; 



but others live in colonies, each newly formed cell remaining 

 attached to its parent cell, until at last a comparatively large com- 

 pound protozoon is formed. 



The sub-kingdom is divided into several classes, the principal of 

 which, together with their leading characteristics, are shown in the 

 following table : 



1. Rhizopods : Body uniform in consistence. 



Pseudopods protruded from any point. 



2. Protoplasta: Outer protoplasm slightly firmer in consis- 



tence. 



Pseudopods protruded from any point. 

 (Often grouped with the Wiizopods.) 

 3 Eadiolaria : Possessing a central membranous capsule. 



Usually supported by a flinty skeleton. 

 4. Infusoria : Outer protoplasm firmer and denser ; therefore 



of more definite shape. 



Possess permanent threadlike extensions of protoplasm 

 instead of pseudopods. 



We shall now observe the principal marine members of the 

 protozoa, commencing with the lowest forms, and dealing with 

 each in its proper zoological order as expressed in the above table. 



MARINE RHIZOPODS 



When we stand on a beach of fine sand on a very calm day 

 watching the progress of the ripples over the sand as the tide recedes 

 we frequently observe whitish lines marking the limits reached by 

 the successive ripples as they advance toward the shore. If, now, 



