PHYSIOLOGY OF THE INVERTEBRATA 343 



whereas it is situated ventrally in the Arthropoda, Never- 

 theless there is a certain amount of homology between the 

 spinal cord of the cranial and acranial Vertebrata on the one 

 hand, and the ganglionic chain of the Arthropoda on the 

 other.. In both the Vertebrata and the higher Invertebrata, 

 there is a special nervous network, which supplies the 

 alimentary canal, the respiratory and urino-genital organs, 

 and the circulatory system. In both of these sub-kingdoms 

 this system has its origin, or at least its roots, in the great 

 nervous centres. 



In concluding these remarks, it may be stated that on the 

 whole, every nervous system, whether Invertebrate or Verte- 

 brate, resolves itself into a number of cells, and into a number 

 of fibres, which connect the cells or terminate therein. The 

 parts of the system where the cells accumulate in great 

 number are the nervous centres. The parts are almost wholly 

 composed of fibres from the nervous cords, and if we look 

 at the animal kingdom as a whole, we see that where the 

 cellular centres are the more voluminous and the less 

 numerous, the higher the animal is in the zoological scale. 

 In fact, the mammal has been said to be "a sort of summary 

 of the entire kingdom. In him are combined all the tissues, 

 all the apparatus scattered through the entire series : he has 

 a special nervous system, but he possesses, nevertheless, a 

 portion of the ganglionic system of the Invertebrates, and 

 in him, as in them, this ganglionic system is constituted 

 essentially of fibres," derived in the first instance from a 

 protoplasmic basis. 



The Tunicata. 



The nervous system of these animals consists of an 

 elongated cerebral ganglion situated on the dorsal side of the 

 pharynx. Nerves are given off from this ganglion to the 

 entrance of the pharyngeal sac, &c. ; nerves are also sent out 

 laterally and posteriorly. In the Ascidian larva the nervous 



