. Chordate morphology. Morphology (Animals); Chordata. 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. Ther
. Chordate morphology. Morphology (Animals); Chordata. 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
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