SENSE-ORGANS OF THE INTEGUMENT 253 



can be recognised, and along the sides of the body and tail 

 are one or more lateral lines 1 (Figs. 185 and 186). These struc- 

 tures are thus often spoken of as segmcnial sensory organs, or wgans 

 of the lateral line' 2 : primarily they have not a metameric arrange- 

 ment, and where such is seen, it is always secondary. The portions 

 lying in the region of the head are the first to be developed. They 

 are innervated by the lateral line branches of the facial, glosso- 

 pharyngeal and vagus (cf. pp. 242, 245). 



It is thus clear that the entire lateral nervous system and its 

 modifications, including the auditory organ, is a specialised system 

 differing morphologically and histologically from all the other 

 integumentary sense-organs. 



The so-called Savi's vesicles of Torpedo, the nerve-sacs or 

 pit-organs of Teleostomes, and the ampullary tubes of Elasmo- 

 branchs correspond to modified nerve-eminences. These are 





FIG. 186. DISTRIBUTION OF THE LATERAL SENSE-ORGANS IN A SALAMANDER 



LARVA. 



all limited in their distribution to the head and anterior portion 

 of the trunk, being most numerous on the snout : they arise from 

 thickenings of the epiderm which later become invaginated and 

 in which a sensory epithelium is differentiated. In Teleostomes 

 these organs retain a simple sac-like form, and are situated 

 abundantly on the head and along accessory lateral lines on the 

 trunk : in Torpedo they become completely separated from the 

 epiderm, while in other Elasmobranchs they are tubular, each 

 tube giving rise to one or more swellings or ampullae of varied 

 form enclosing radial folds of connective tissue and containing the 

 nerve-end organs, which are supplied by the lateral-line system 

 of nerves. The tubes are filled with a gelatinous substance. 



These integumentary sense-organs are extremely ancient 

 structures, for traces of them have been observed in Jurassic 

 Elasmobranchs, and even in the Devonian CephalaspidaB and 



1 There are several lateral lines in Polypterus and various other Fishes, in 

 Proteus, and in all amphibian larvje. 



- The lateral canal system of Polyodon comes nearest to that of Elasmo- 

 branchs, and in Acipenser it shows certain resemblance to that of Bony Ganoids, 

 in that the sensory organs become embedded within cranial elements. In Lepi- 

 dosteus, branched secondary canals arising from the main canals of the head 

 extend into the cranial bones : this is not the case in Polypterus. In Teleosteans 

 the system is very different from that of Elasmobranchs, but resembles that of 

 Ganoids in many respects e.g. in often having the canals protected by bony 

 structures : in other respects the different families and species differ much from 

 one another, and reductions of the lateral line organs may occur (e.g. in 

 Siluroids). 



