INTRODUCTION. 47 



VARIETIES OF ANIMAL ORGANIZATION. 



Nature it seems has constructed the animal kingdom upon 

 four great models or types. 



1. The Kadiata forming the zoophytes. 



2. The Articulata worms, insects, &c. 



3. The Mollusca shell-fish, &c. 



4. The Vertehrata having a spine. 



The Eadiata, so called from their resemblance to a ra- 

 diated flower or star, are of simple structure, and described 

 as having no distinct nervous system, TIO organs of sense, 

 no heart, and having white blood. 



This class includes the polypus, hydratid, coral, sponge, 

 star-fish, infusoria, &c. 



The Articulata are a step higher in organization, for here 

 are found nervous ganglia, forming a longitudinal chain 

 along the median line of the body. The body itself is di- 

 vided into rings, which feature gives the name to the Class, 

 and has a protection or kind of skeleton exteriorly of hard- 

 ened skin, or horny covering, as seen in the insect and lob- 

 ster. The blood is also generally white, and there is no 

 heart, but simply a vessel running along the back called 

 the dorsal vessel. There are senses, but they are more or 

 less incomplete. 



This Class includes the Crustacea, as the Crab, &c., the 

 Arachnides or Spiders, the Annelides or Worms, and In- 

 sects. 



The Mollusca, as their name implies, have the body soft, 

 and like the Articulata, have nervous ganglia ; but, instead 

 of being united, these are found scattered throughout the 

 body, and not in a chain along the middle line. The senses 

 are also incomplete, the blood, white; but here we find a 

 heart. 



There are no rings, no external skeleton, but simply a 

 stony crust or covering constituting the shell. 



