THE SENSE-ORGANS 469 



which branch repeatedly and with their ultimate fibers form 

 a delicate net-work which permeates the entire layer, stopping- 

 only at the most external cells. This type is characteristic of 

 all vertebrate integument, including both cyclostomes and 

 mammals, and furnishes them all with a surface capable of re- 

 sponding to general tactile impressions. 



Aside from this general tactile sense vertebrates possess a 

 large number of more or less specialized sensory endings, 

 usually classed also as tactile. These are local in distribution, 

 often confined to a single group of animals, and usually oc- 

 cur upon prominent portions of the body or occasionally, as 

 in the case of certain fishes, upon special papillae or long fila- 

 ments. The most extensive of these, and the only one to 

 develop into a definitely organized system, is that of the lateral 

 line organs, referred to above in connection with the cranial 

 nerves and possessed by the primarily aquatic vertebrates 

 (fishes and amphibians). Although this system, as such, to- 

 gether with its nerves, disappears utterly with the assumption 

 of a terrestrial life, it is yet of importance in this connection 

 because of the possible derivatives from it in higher forms, 

 among which have been mentioned, with more or less basis 

 for the claim, the taste-buds, the inner ear, and the mammalian 

 hair. 



In its simplest form a lateral line organ consists of a small 

 group of sensory cells, slightly convex in form and protected 

 by a wall of non-sensitive supporting cells. This organ gains 

 its simplest form of protection by sinking slightly beneath 

 the surface, its supporting cells remaining at the general level, 

 or even projecting a little above it. By continuing this process 

 the sense-organ comes to lie at the bottom of a flask-shaped 

 cavity, communicating with the surface by a narrow neck. 



From this point on, greater complexity may be gained by 

 development in one of two directions, the flask-shaped cavities 

 may either become associated in rows and break down their 

 adjacent walls, forming the slime canals, or else each separate 

 flask may become elongated, bearing the sense-organ at its 

 very bottom, as in the canals of Lorenzini. In the first of 

 these, the coalescence may result in the formation of either a 



