. Chordate morphology. Morphology (Animals); Chordata. ^^ temporal ^ia^placode 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 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 bot
. Chordate morphology. Morphology (Animals); Chordata. ^^ temporal ^ia^placode 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 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
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