ORGAA T S. 



6 7 



FIG. 67. Lateral line organ 

 of Amblystoma, showing, be- 

 neath, the nerve fibres ; on the 

 free surface the sensory hairs. 



terrestrial vertebrates where the surface of the skin is dry, the 

 sensory structures sink to a deeper position. 



Lateral Line Organs. Some of these organs are irregularly 

 distributed, while others are grouped into regular series, and 

 form what are known as the lateral 

 line organs. In their early stages 

 these lateral line organs are upon 

 the surface. Later they sink, in the 

 amphibia, into pits, in pisces into lon- 

 gitudinal grooves which may be closed 

 into tubes, with openings at regular 

 intervals. With increase in size of 

 the animal, the number of openings 

 also increases by division. The open- 

 ings frequently perforate scales, while 



the canals between them may become enclosed in bone, espe- 

 cially upon the head. By the presence of grooves and canals 

 in the skulls of many fossil forms, we infer that they pos- 

 sessed lateral line organs. There is considerable variation in 

 the distribution of the lines of these organs, but the following 



are the most constant 

 series : (i), the lateral 

 line of the trunk (may 

 be double) which ex- 

 tends the length of the 

 body between the dorsal 

 and ventral musculature ; 

 this series gives the name 

 to the whole system; (2), 

 occipital series, crossing 



FIG. 68. Dorsal view of head of sturgeon, 

 showing the distribution of the lateral line canals, 

 after Collinge. de, dermethmoid ; do, dermo- 

 occipital ; ee, dermoectethmoid ; ep, epiotic; 

 //', frontal; m, main canal; oc, occipital com- 

 missure; pa, parietal; //, post-temporal; so, 

 suborbital canal; spo, supraorbital canal; sg, 

 squamosal. 



the back of the head and 

 connecting the systems 

 of the two sides ; (3), 

 supraorbital, and (4), in- 

 fraorbital series, running respectively above and below the eye ; 

 (5), mandibular series, upon the lower jaw. In these grooves 

 or canals are the groups of sensory cells, the groups on the head 

 being innervated by the ophthalmicus superficialis, bticcalis and 



