CHAPTER Vn. 



DATA CONCERNING THE BRAIN DERIVED FROM THE STUD-J 

 OF THE NERVOUS SYSTEM OF INVERTEBRATES. 



This survey of some of the principal varieties of the 

 Nervous System among the Invertebrata, brief though it 

 has been, should have sufficed to call attention to many 

 important facts and to show the warrant for certain related 

 inferences, many of which are embodied in the following 

 propositions : 



1. Sedentary animals, though they may possess a 

 Nervous System, are often headless, and they then have 

 no distinct morphological section of this system answering 

 to what is known as a Brain. 



2. Where a Brain exists, it is invariably a double 

 organ. Its two halves may be separated from one 

 another ; though at other times they are fused into what 

 appears to be a single mass. 



3. The component or elementary parts of the Brain 

 in these lower animals are Ganglia in connection with 

 nerves proceeding from special impressible parts or Sense 

 Organs ; and it is through the intervention of these united 

 Sensory Ganglia that the animal's actions are brought into 

 harmony with its environment or medium. 



4. That the Sensory Ganglia, which in the aggregate 

 constitute the Brain of invertebrate animals, are connected 



