398 



CHORDATE ANATOMY 



A sense organ of the lateral line consists of a cluster of pear-shaped 

 hair cells, each of which is connected with a branch of the lateral-line nerve. 

 Usually the cluster of sense cells is encircled by a ring of columnar epi- 

 thelial cells. The sensory cells of neuromasts, unlike those of taste-buds, 

 do not extend to the base of the epithelium. 



It is generally assumed that neuromasts have evolved from clusters of 

 neurosensory cells like those of lower invertebrates. Similar clusters 

 occur in the oral tentacles of Amphioxus. Later in phylogenesis, however, 

 sensory cells of the secondary type become the sensory elements of the 

 neuromasts. How this change occurred is problematic. 



Fig. 350. — Head of pollack, showing lateral-line canals and nerves of the lateralis 

 system. Lateralis nerves black, canals and brain dotted, b, buccalis ramus of VII 

 nerve; dl, dorsal ramus of lateralis of X nerve; h, hyomandibularis nerve; hm, hyo- 

 mandibular line of organs; io, infraorbital line; I, lateral-line canal; n, nares; o, olfactory 

 lobe; op, operculum; os, ophthalmicus superficialis nerve; soc, commissure connecting 

 lines of the two sides; so, supraorbital line of organs; st, supratemporal part of lateral 

 line; vl, ventral ramus of lateralis of X nerve; x, visceralis part of X nerve. (From 

 Kingsley's "Comparative Anatomy of Vertebrates," after Cole.) 



The stages in the evolution of neuromasts are repeated in ontogenesis 

 and also represented in the lateral-line organs of adult craniotes. Pri- 

 marily, the neuromasts are scattered over the surface of the body, but 

 appear first in the head region. Those which become components of 

 the lateral-hne organs sink into grooves, which extend from the ear region 

 both craniad and caudad. There is evidence that neuromasts originally 

 had a metameric arrangement, but this metamerism is usually lost through 

 the subdivision and multiplication of the primary clusters of sensory cells. 



In cyclostomes and tailed amphibians each neuromast sinks into a 

 separate pit. In the fishes lines of neuromasts sink below the surface. 

 In Chimaera the grooves remain open through life. In other groups of 



