ZOOLOGY OF THE INVEKTEBRATA. 



295 



present, which, as a rule, are only found in plants, so that it 

 becomes at once evident that the line between animals and plants, 

 at any rate in their lowest forms, represents no scientific frontier, 

 but is an arbitrary boundary which is apt to be shifted, now for- 

 ward, now backward, according to the opinion of the investigators." 

 The" volume is divided into twenty-one chapters, each treating 

 of a separate class of the Animal Kingdom, commencing with 

 Protozoa, Metazoa, Coelenterata, etc., up to Insecta, Arachnida, 

 and Chordata. 



Fig. 73. — Proterospongia Haeckeli^ Sav. Kent, x 800. 



1. Nucleus. 



2. Contractile vacuole. 



3. Collar. 



4. Flagellum. 



5. Amoeboid individual sunk in sup- 



porting jelly. 



6. Other individuals undergoing tission 



7. Individual with collar contracted. 



8. Individual divided up into number 



of spores (microgonidia). 



9. Jelly-like supporting matrix. 



The Protozoa have been divided into two groups : the Gymno- 

 myxa, corresponding with the old group, Rhizopoda, and the 

 Corticata, which comprise the Infusoria and Ciregarinidea. We 

 shall select our tirst illustration from the group Corticata. These 

 animals have, as a common feature, a differentiation of the pro- 

 toplasm into a more fluid central portion and a firmer cortical 



