LATERAL-LINE ORGANS 



195 



cuticular hairs or bristles on their free ends (fig. 201), while the 

 deeper ends are embraced by the non-medullated fibrils of the later- 

 alis system of nerves, which follow the lines of organs, and in develop- 

 ment keep pace with their extension. These sensory areas are the 

 nerve hillocks or neuromasts already referred to (p. 181). 



Fig. 201. Fig. 202. 



Fig. 201. — Sense organ of lateral line of Diemyctylus (aquatic form) freely after 

 Kingsbury, c, cone cells; s, spindle cells. 



Fig. 202. — Developing lateral line organ on one side of head of Atnia, showing 

 method of closure of grooves to canals, after Allis, an, anterior naris; to, so, infra- 

 and supraorbital lines; pn, posterior naris. 



In the cyclostomes and aquatic amphibia each sensory patch 

 sinks into a separate pit (fig. 201), but in all other ichthyopsida the 

 lines of organs sink in the same way, the patches being connected by 



Fig. 203. — .■stereogram oi i.iicrai-iinc organs of a nsh. c, lateral-line canal; In, 

 lateralis nerve; p, pores connecting with the exterior; s, scales in skin; so, sense organs of 

 lateral line. 



grooves. In Chimcera these grooves remain open, but in all others 

 they are closed except at certain points where pores connect the 

 canals formed by the closed grooves with the exterior. In this way 

 the sensory areas come to lie in canals beneath the surface, water 



