3 6 THE POPULAR SCIENCE MONTHLY. 



into correlated action witb. the jaws. The posterior portion of the 

 sub-oesophageal mass receives nerves from, and also gives off nerves 

 (14) to, the branchiae and other viscera, as well as to the mantle (13, 13). 



The auditory organs and their nerves are also connected with this 

 branchial and pallial ganglion. These organs are lodged in the sub- 

 stance of the cartilaginous framework investing the nerve-ganglia a 

 structure which seems to answer to a rudimentary skull. The roots 

 of the auditory nerves are probably principally in relation with the 

 pallial portion of the branchio-pallial ganglion. The locomotions of 

 these creatures are largely brought about by contractions of the pal- 

 lial chamber, though these contractions of the mantle are also subser- 

 vient to the respiratory function. 



The share which the branchio-pallial ganglia take in bringing 

 about and regulating the movements of the cuttle-fish would seem to 

 explain the connection of the auditory nerves with them rather than 

 with the homologues of the pedal ganglion, with which the auditory 

 saccules are in relation in most other mollusks. But, whatever may 

 be the precise explanation of the different connections of the auditory 

 nerves in the cuttle-fish tribe, the fact remains that their connections 

 are still away from the brain proper. They are, as in most other 

 Mollusca and in those insects in which auditory organs are known to 

 occur, in intimate relation with one of the principal motor centres. 



This survey of some of the principal forms of the invertebrate 

 brain, brief though it has been, should have sufficed to call attention 

 to the following important facts and inferences : 



1. That sedentary animals, though they may possess a nervous 

 system, are often headless, and then have nothing answering to a 

 brain. 



2. That where a brain does exist, it is invariably a double organ. 

 Its two halves may be widely separated from one another, though at 

 other times they are fused into a single mass. 



3. That the component or elementary parts of the brain in these 

 lower animals are ganglia in connection with some of those special 

 impressible parts or sense-organs, by means of which the animal is 

 brought into harmony with its environment or medium. 



4. That the sensory ganglia, which as an aggregate constitute the 

 brain of invertebrate animals, are connected with one another both 

 on the same and on opposite sides of the body, either by continuous 

 growth or by means of commissures. 



5. The size of the brain as a whole, or of its several parts, is 

 strictly regulated by the development of the animal's special sense- 

 organs. This is so, because, the more these impressible surfaces be- 

 come elaborated and attuned to help in discriminating between nu- 

 merous different external impressions, the larger are the ganglionic 

 masses with which their nerves ai*e in relation. 



6. Of the several sense-organs and sensory ganglia whose activity 



