142 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



connective- tissue compartment with a gelatinous substance between 

 them and all with their nervous layer turned in the same direction. 



In Torpedo the organ apparently is derived from part of the jaw 

 muscles and the prisms of plates are arranged vertically. In Astro- 

 scopus (fig. 150) it is said that the tissue comes from some of the 

 eye muscles, while in Gymnotus the ventral trunk muscles are con- 

 cerned and the columns of electroplaxes are horizontal. In the same 

 fish the discharge is always in the same direction, e.g., in Torpedo 

 from below upward. 



THE NERVOUS SYSTEM 



Nervous and sensory structures are closely related to each other, 

 and their distinction in the higher animals is the result of differentia- 

 tion among cells which were originally both nervous and sensory in 

 character, and it is in this broader sense that the term nervous 

 structures is used in these introductory paragraphs. 



The nervous system primarily has to inform the animal of the con- 

 ditions, good and bad, in the environment, to correlate this informa- 

 tion and to regulate the motions so that advantage may be had of this 

 knowledge. These facts have determined several features of the 

 nervous system. Thus they have determined its origin in the ecto- 

 derm, the outer layer of the body, which comes into relation with the 

 external world. Since this information has to be carried to internal 

 parts, conducting tracts or nerves have arisen, while the correlating 

 function has been localized in the body of the cells where incoming 

 and outgoing tracts meet. 



Very important of the primitive functions was the determination 

 of the character of the food, which would lead to the greater aggrega- 

 tion of the nervous tissue around the mouth. As we have seen (p. 

 12) the anlage of the central nervous system of the vertebrates oc- 

 cupies such a position around the blastopore, or mouth of the gastrula, 

 in the form of the neural plate. As the external surface of the body is 

 most exposed to injury, the nervous structures, with the closure of 

 the blastopore, have been protected by removal to a deeper position, 

 through the rolling of the plate into a tube. The closure of the 

 blastopore brings the two halves of the plate into close association 

 with each other, making it a bilateral structure. With bilaterality 

 comes the tendency of one end of the animal to take the lead, re- 

 sulting in the concentration of nervous and sensory structures at the 

 anterior end, which first comes in contact with foreign objects. In 



