150 THE HUMAN SPECIES 



ends are situated at the mouth or orifice for the admission of 

 food, and in accordance with the radial structure are likewise 

 radial. These radial nerves surrounding the mouth have 

 been named by Joh. Miiller "ambulacral brain," and the term 

 fitly describes the structure as compared with the true brain of 

 the higher animals. With still more justice can we speak of a 

 primitive brain in the Arthropoda, whose nervous system cor- 

 responds on the whole to that of the Annelida, for of the two 

 pharyngeal ganglia connected by commissures, the upper, or 

 head ganglion, is always of more importance than the lower. 

 Proceeding from the two pharyngeal ganglia, there is a chain 

 of abdominal ganglia arranged in pairs and connected by com- 

 missures ; sometimes, by means of transverse coalescence, these 

 ganglia form an abdominal cord, corresponding to the spinal 

 cord of the Vertebrates, and send off nerve fibres for the muscles, 

 intestines and skin. 



In the insects the beginnings of a sympathetic system are 

 evident, the system taking its rise in the abdominal ganglia and 

 connected otherwise with the usual nerve fibres. 



In the Mollusca also a sympathetic system is present. At 

 first sight their nervous system appears extremely complex, 

 especially in the Bivalves, but closer examination reveals the 

 fact that it is constructed on the same plan as that of the 

 worms. It consists of two upper and two lower pharyngeal 

 ganglia connected by commissures, and various distinctions are 

 present, particularly with regard to the peripheral nervous 

 system. As a rule, the nerve fibres proceed from the central 

 parts of the pharyngeal ring and are connected with one another, 

 and also with various independent ganglia. 



Distinctively characteristic of the Invertebrates are the gan- 

 glia surrounding the oral aperture, and representing in their re- 

 lation to the other ganglia a primitive order of central nerve 

 organ, and distinctive of the Vertebrates is the symmetrically 

 arranged nerve matter of undoubtedly central character enclosed 

 in bony casement. In the Acraniata the nerve substance is dis- 

 tributed equally throughout the entire length of the body ; in 

 the Craniata it is divided into brain and spinal cord, but in all 

 the Vertebrates alike the nerve masses originate in the upper 

 blastoderm, first three and subsequently five consecutive cerebral 



