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COMPARATIVE ANATOMY 



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 line organs sink into grooves, which extend from the ear region 

 both craniad and caudad. There is evidence to suggest 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 amphibia each neuromast sinks into a 

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

 In chimaera the grooves remain open throughout life. In other groups of 



Fig. 469. — Stereogram of lateral-line organs of a fish, c, lateral-line canal; In, 

 lateralis nerve; p, pores connecting with the exterior; 5, scales in skin; so, sense organs 

 of lateral line. (From Kingsley's "Comparative Anatomy of Vertebrates.") 



fishes, this being the definitive condition, the grooves are converted into 

 mucus-filled canals with occasional pores opening to the surface. 



Lateral line organs, although limited to the ichthyopsida, are of 

 special interest to morphologists since the evidence leads some to believe 

 that the nose and ear are modifications of these. The reasons for this 

 conclusion are more convincing for the ear than for the nose, since the 

 hair cells of the nose resemble primary neurosensory cells of the inverte- 

 brate type, not the secondary sensory neuromast cells characteristic of 

 lateral line organs. 



Development op Lateral Line Organs 



In fish embryos, the first indication of lateral line organs is a local 

 thickening of the ectoderm in the ear region above the first visceral 

 pouch. By local thickening the placode becomes extended anteriorly 

 and posteriorly. Later stages resemble those of phylogenesis already 

 described. Differentiation of the canals proceeds both craniad and 

 caudad. Grooves are formed in the elongated placode, which later close 

 over, leaving apertures at regular intervals. The epithelial cells at the 



