THE OEGANS. 41 



as a sensory organ^ for wticli reason sucli an organ must be at first 

 placed superficially. This superficial position of tlie earliest rudi- 

 ments of the nerve-centre has been already made out in so many 

 forms that it may be regarded as a general pheenomenon. As the 

 sensory organ becomes differentiated from the ectoderm it sinks 

 down into the body. The developing central organ is thus 

 gradually covered over by other layers of the body. This 

 arrangement^ which is most peculiar, and by itself most unin- 

 telligible, is explained if we regard it as inherited from a more 

 primitive stage, in which the nervous system was but slightly 

 differentiated, and was represented by the whole cell-layer of the 

 ectoderm, or by part of it. We must consider its gradual attain- 

 ment of an internal position to be a process due to its continued 

 differentiation, and consequent higher potentiality ; the organ, which 

 has become of greater value to the organism, gets hidden within the 

 body. 



With regard to the structural characters of the differentiated 

 nervous system, the central organ, which is chiefly composed of 

 ganglion- cells, is to be distinguished in the first place from the 

 nerves, which pass to the terminal apparatus, and consist of fibrous 

 elements (peripheral nervous system). 



§ 35. 



The earliest complications are due to the appearance of several 

 parts (ganglia), in which are central form-elements connected with 

 one another : the further development of these parts is very various. 

 The ganglionic mass, which forms the central organ, is primitively 

 dorsal, owing to the earliest separation of the central organs taking 

 place from the dorsal ectoderm, as we have already seen. This 

 dorsal nervous mass, which generally lies near the entrance to the 

 alimentary canal, is differentiated into several parts, which are con- 

 nected together by commissures; their fibres form an oesophageal 

 ring. 



In the animals built on a radiate plan the number of the ganglia is 

 increased in correspondence with the radii ; the peripheral distribution 

 of the nerves also follows these general structural relations exactly. 

 The nervous system in bilaterally-symmetrical animals follows the 

 bilateral arrangement. The more primitive form is represented by 

 a superior ganglionic mass (cerebral ganglion). Other ganglia do 

 not seem to be formed until the metameres are formed. We then 

 are able to distinguish dorsal and ventral ganglia ; the latter may form 

 ganglionic masses along a continuous longitudinal trunk, or a single 

 subcesophageal ganglion. The variations in size of these oesophageal 

 ganglia are in the closest connection with the nerves which pass off 

 from them. When sensory organs are developed, the ganglion 

 which sends off their nerves becomes of considerable size, while it 

 seems to degenerate when they grow less. The supra-oesophageal 

 ganglia are the most important in this relation, for it ia from 



