the bone. In Drepanaspis these canals are represented partly 

 by grooves and partly by canals. In most Osteostraci, ex- 

 cept Tremataspis and closely related genera, the reticular 

 canals are indicated by interareal grooves, and the sensory 

 lines by short canal segments as well as grooves. 



The pattern of sensory lines as seen in Tremataspis can be 

 taken as representative of this group. The supraorbital line 

 is reduced to a small segment behind the eye. The infraor- 

 bital canal is separated by a gap from the main or ventral 

 lateral lines. There is a postorbital canal and a supraoccipital 

 canal extending dorsally to the endolymphatic pore and 

 ventrally to the ventral lateral line, which is divided into 

 parts above and below the ventrolateral fold. Two lines oc- 

 curring below the branchial openings can be viewed as con- 

 tinuations of the postorbital line and ventral lateral lines. 

 There is a short section of dorsal lateral line to either side 

 of the dorsal spine of the trunk. 



The canal system of the heterostracan is much less modi- 

 fied since the eyes are lateral and the branchial chambers 

 are not crowded below the cranium (Figure 13-25 B). A 

 supraorbital canal extends forward on the snout and back 

 to a point behind the pineal organ. There are dorsal, lateral, 

 and ventral body lines. The postorbital canal joins the 

 lateral and ventral lines behind the eye and continues on 

 the throat, behind the mouth, as the oral canal. The ventral 

 line continues forward beyond the postorbital line as the 

 infraorbital line. There is no hyoid line. There are several 

 connectives on the trunk, behind the level of the ear, be- 

 tween the dorsal and main lines. The most anterior of these 

 probably represents the dorsal end of the hyoid line. The 

 next is the dorsal end of the first branchial line, the middle 

 pit line of higher forms. The posterior one appears to be the 

 occipital canal. 



Of the living agnaths, the cyclostomes, only the lamprey 

 has an evident lateral-line system (Figure 13-26). There are 

 superficial sensory organs (neuromasts) forming a temporal 

 line, a supraorbital line represented by two organs, an in- 

 fraorbital line not continuous with a line on the snout, and 

 a line below the branchial chambers. There is also a ver- 

 tical line passing downward parallel to the margin of the 



mouth. These lines are innervated by the seventh nerve. 

 There are indistinct dorsal, lateral, and ventral body lines 

 as well as organs behind and above the branchial openings. 

 In some specimens there is a small hyoid clump of organs. 

 The pattern observed in the lamprey agrees with that of the 

 fossil forms. 



From the agnaths one could conclude that the ancestral 

 vertebrate lacked a sensory-canal system or that it had an 

 extensive reticular system in which certain channels came 

 to be the primary sensory hnes. Either of these diametrically 

 opposite views is reasonable, as is the possibility that the 

 primitive state was somewhere in between. The reticular 

 and sensory-line systems may have originally been separate 

 (as in the crossopterygian fishes?), the former functioning in 

 mucous production, protecting the body both from abrasion 

 and from water penetration. 



In a well-developed sensory canal system, in terms of 

 primary lines distinct from the reticular system, three pri- 

 mary pairs of longitudinal lines can be distinguished. These 

 are connected by a single canal, the postorbital. This pat- 

 tern can only in part be compared with that of the 

 gnathostome groups. 



Shark Of the sharks the most primitive representative is 

 thought to be Chlamydoselachus, which closely resembles the 

 hypothetical ancestral gnathostome pattern (Figure 13-25 C). 

 Here the main lateral line joins the temporal canal, and the 

 dorsal line is interrupted on the cranial roof The supra- 

 orbital line loops forward on the snout and connects with 

 the infraorbital behind the nasal capsule. The infraorbital 

 line continues forward beneath the nasal capsule; the lines 

 of either side touch below the tip of the snout and are re- 

 flexed back a short distance to end blindly. The jugal por- 

 tion of the mandibular arch line passes back to the angle of 

 the mouth, where it turns down and forward as the oral line. 

 This line extends nearly to the tip of the lower jaw, where 

 it ends blindly or occasionally joins the canal of the oppo- 

 site side. The jugal line has an extension which continues 

 back nearly to the hyoid arch line; this is probably a section 

 of the ventral line. The cheek portion of the hyoid arch line, 



nasohypophyseal opening 

 supraorbital line 



temporal line 



occipital line 



dorsal line 



•Xr^ f 



» # 



"lateral line- 



infraorbital line 



branchial arch lines 



• • 



hyoid arch line 

 Figure 13-26. Distribution of sensory-line organs and grooves in the lamprey. 



412 • THE NERVOUS SYSTEM 



