THE SENSE ORGANS 



569 



of the glossopharyngeal nerve. A supratemporal line may connect the 

 systems of the two sides across the posterior part of the skull. 



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. 



Fig. 468. — 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.) 



Evolution of Lateral Line Organs 



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. At present, 

 there is no evidence adequate to settle the question whether this change 

 involves the replacement of primary sensory cells by secondary ones, 

 or by the retention of the cell-bodies of the primary cells and the substitu- 

 tion of new nervous connexions for the primary ones. Since neuromasts 

 with secondary sense cells like those of chordates occur also in leeches 

 of the genus Glossosiphonia, it is evident that the substitution of secondary 

 for primary neurosensory cells has occurred independently in vertebrates 

 and invertebrates. 



