B 



posterior pit line 

 middle pit line 

 antertor pit line^ 

 postorbital 



occipital line 



premoxillary arch lines 

 (supraorbital) 



ventral sensory line 



middle pit line 

 anterior pit line 

 pineol body 

 postorbital 



supraorbital line^ 



posterior pit line 



1 endolymphatic pore 



JO 



infraorbital line 



mandibular arch line 



hyoid arch line 



anterior pit line 

 spiraculor line 

 pineal body 



middle pit line 



posterior pit line 



endolymphatic pore 



ventral sensory line 



supraorbital line 

 nasal capsule 



dorsal line 



lateral line 



infraorbital line 



mandibular arch line 

 and pit line 



hyoid arch line and accompanying pit line ventral sensory line 



Figure 13-25. A, arthrodire; B, primitive vertebrate; C, Chondrichthyes, and Osteichthyes. 



In the heavily armored forms these canals grooved the sur- 

 face of the bone (interareal grooves) or were buried in the 

 bone as in the case of Tremataspis. 



It was early noted that there were connections between the 

 reticular and sensory-line canal systems, and in fact one is 

 unable to distinguish between "mucous" and sensory canals 

 in Tremataspis, as they are seen in section, for they appear to 

 be parts of a single system. The canals in this genus are di- 

 vided into outer and inner parts by a thin perforated bony 

 septum (Figure 8-26). 



Among the agnaths there is a great deal of variation in 

 this system ranging from Irregulariaspis and Tremataspis, hav- 

 ing a network of canals in which the sensory lines are 

 straighter and have slit-like openings to the surface of the 

 bone, to most forms having only the sensory lines. In many 

 of the Heterostraci, only the sensory canals are embedded 

 in the bone, but a pattern of interareal grooves suggests the 

 presence of a more superficial reticular system. In some of 

 the heterostracans, the astraspids and amphiaspids, only 

 the sensory canals are indicated by superficial grooves in 



SENSORY ORGANS 



411 



