. Comparative anatomy. Anatomy, Comparative. 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 Fig. 468.—Head of pollack, showing lateral-line canal
. Comparative anatomy. Anatomy, Comparative. 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 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 Glossosiph
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