SENSE-ORGANS 19 



another important method of describing the nervous system, taking 

 account not so much of the segmental value of the nerves as of 

 their peripheral destination and central connections. For this 

 purpose, the sense-organs must first of all be considered. 



Scattered sensory cells alone are found in the skin of Amphioxus, 

 and small simple sense-organs on the buccal cirrhi. The Craniate 

 vertebrates, on the other hand, especially the Gnathostomes, are 

 provided on the outer surface of the body and on the inner surface 

 of the alimentary canal with a network of free nerve-endings, and 

 a variety of sense-organs the structure, distribution, and nerve- 

 supply of which have been admirably worked out in the lower 

 vertebrates by numerous anatomists, whose results are of consider- 

 able interest for the study of phylogeny (Schulze [391-2], Leydig 

 [283], Strong [428], Allis [9, 10], Johnston [247-9], Herrick [210], 

 Ewart [133, 134], Cole [82], and many others). 



In all the lower aquatic Craniata we find an important series 

 of sense-organs on the head and trunk constituting the 'lateral- 

 line system' (Figs. 11, 85). Possibly they were primitively strictly 

 metameric, as they are now in some fish ; at all events, these sense- 

 organs (neuromasts) have a definite distribution and nerve-supply, 

 and become of great taxonomic value in the Gnathostomes. The 

 less regularly arranged 'pit-organs,' ampullae, etc., of fish appear 

 to be related to the lateral-line system. All the nerve-fibres 

 derived from these lateral-line organs on the head enter the brain 

 by the facial, glosso-pharyngeal, and vagus nerves (and probably the 

 profundus also in Petromyzon}. The lateral line of the trunk is 

 supplied exclusively by the ramus lateralis vagi. Moreover, it has 

 been shown to be extremely probable (Beard [31], Ayers [22]) that 

 the ear and the auditory nerve represent a highly differentiated 

 portion of the same system. The whole forms the ' acustico-lateral 

 system,' whose nerve-fibres terminate centrally in the tuberculum 

 acusticum of the medulla and associated centres. 



That a cranial nerve, the vagus, should supply a series of sense- 

 organs reaching to the tip of the tail strongly suggests that the 

 lateral-line nerve is a collector, similar to the epibranchial nerve. 

 Moreover, it has been observed (Alcock [7]) that in the branchial 

 region of the larva of Petromyzon there are a series of lateral 

 metameric groups of such organs, each supplied by a twig from the 

 branchial nerve of its own segment (this needs confirmation). 

 Evidence of a segmental origin may also be found in the develop- 

 ment of the lateral-line organs in Petromyzon. The nervus lateralis 

 vagi arises from a longitudinal thickening of the epiblast above the 

 dorsal ganglia, which is continued on the head as a series of 

 dorso-lateral ' placodes ' contributing to the formation of the ganglia 

 of the 9th, 7th, 5th, and profundus. Similar placodes occur in the 

 lower Gnathostomes. On the other hand, the independence of the 



