^^ temporal 

 ^ia^placode 



7.5 1 



B 



8 mm 



middle pit line 

 intertemporal organ 



anterior pit line 

 supraorbital tine 



nasal opening 

 infraorbital line 



posterior pit line 



posttemporal organ 



tabular organ 

 suprotemporal 

 mandibular arch line 



oral sucker 



mouth 



6.75 mm 



Figure 13-31. Development of the sensory-canal system of the head in actinopterygians. A and B, 

 two stages in Amio,- C, an early stage of Po/ypferus. (A and B after Holmgren and Pehrson, 1949; 

 C after Pehrson, 1958) 



Choanatei 



CROSSOPTERYGIAN In the early crossopterygians and dip- 

 noans both reticular and sensory-line canals were present 

 and enclosed in the dermal bones (PoroUpis, Osteolepis). In 

 the later forms (Holoptychius, Eusthenopteron) the reticular 

 system is lacking. 



Of the porolepiforms, the canal system of Holoptychius is 

 best known (Figure 4-32). In this genus there is a typical 

 crossopterygian pattern of embedded canals. The supraor- 

 bital canal is connected with the temporal line and also 

 joins the infraorbital in front of the nasal capsule. The in- 

 fraorbital canals connect from one side to the other on the 

 snout. On the cheek a jugal canal may enter the quadrato- 

 jugal, or this line may be reduced to pit organs. There may 

 be a vertical connection between jugal and preopercular 

 lines. The oral part of the mandibular arch line is made up 

 of pit organs. A separate dorsal piece of the hyoid arch line 

 may extend forward from the spiracle. There are several 

 small pit lines on the roof along the temporal canal — ante- 

 rior, middle, and posterior pit lines. 



The osteolepiform type is well known from both Osteolepis 

 and Eusthenopteron (Figures 4-28, 4-29). The latter retains a 

 part of the reticular system as branched and radiating 

 canals extending to the surface of the bones from the sen- 

 sory line canals. The pattern of the main canals is much 

 like the porolepid, but the mandibular arch line is repre- 

 sented only by horizontal and vertical pit lines. There is 

 also an oral pit line. On the cranial roof are anterior, mid- 

 dle, and posterior pit lines. 



The Actinistia (coelacanths) have a similar sensory-line 

 pattern, and none has reticular canals (Figure 4-33). There 

 has been some modification of the snout canals involving a 

 commissure between supraorbital and infraorbital canals 

 passing behind the nasal openings. A similar commissure is 

 observed in some specimens of Eusthenopteron. 



The crossopterygian can be characterized as having the 

 mandibular arch line represented by irregularly disposed pit 

 lines, by having the hyoid arch line connecting with the 

 postorbital line well down on the cheek, and in having the 

 supraorbital line connecting with the infraorbital in front of 

 the nasal capsule. 



DIPNOAN The primitive dipnoan had an embedded sen- 

 sory line similar to that of the crossopterygian and, in the 

 case of Dipterus, a reticular system (Figure 4-36), which was 

 lost early in this group. The dipnoan differs in that the 

 infraorbital line did not pass forward below the opening of 

 the nasal capsule, or between the internal and external 

 nares, as in the amphibian. 



In living dipnoans a pattern similar to that of larval am- 

 phibians is observed (Figure 13-32). The sensory organs are 

 located primarily in grooves in Protoplerus but in canals in 

 Neoceralodus; these lie in the skin, not in the bones. Protop- 

 terus differs from Neoceralodus in having a connecting line be- 

 tween the mandibular and hyoid arch lines. This groove 

 could represent the jugal extension, turned down rather than 

 up as in the crossopterygian. Whereas the infraorbital line of 

 Neoceratodus is reflexed upward above the nasal capsule, that 

 of Protopterus is bent down and back as if to enter the mouth 

 to pass between the nares. Protopterus has a ventral line on 

 the body as well as a lateral line. 



The development of this system has been described for 

 Neoceratodus (Figure 13-33). The pattern appears to be the 

 same in the other lungfishes and is much like that observed 

 in the shark or Amia. 



Amphibians The amphibians, as derivatives of the choa- 

 nate fishes, have the same general pattern of sensory lines; 

 there is no evidence of a reticular system. The lines are present 

 in the aquatic forms, including the living species, and they are 

 lost in the more terrestrial forms. In the ichthyostegids 

 (Figure 4-25) a part of the canal system is embedded in the 



416 



THE NERVOUS SYSTEM 



