2i8 PHYLUM CHORD ATA 



The Hyoid Arch. The dorsal half of this arch forms the sus- 

 pensorium. It is formed by the hyomandibular and the sym- 

 plectic bones. The former is a large bone which extends from the 

 cranium ventrally, back of the metapterygoid to the symplectic ; 

 the latter is a small bone which lies just dorsal to the hinder end 

 of the quadrate ; the jaws are thus joined with the cranium. 



The ventral half of the arch is formed by a row of bones, on 

 each side, which extend ventrally from the junction of the sym- 

 plectic and the hyomandibular and are called collectively the 

 hyoid bones. These bear the branchiostegal rays. In the middle 

 area between the right and left sides of the hyoid arch is the 

 basihyal bone, which supports the tongue. 



Articulating with both the dorsal and ventral halves of the 

 hyoid arch on each side are bony plates and rods which support 

 structures guarding the openings of the gill slits : these are the 

 opercular bones and the branchiostegal rays. There are four 

 opercular bones : the preopercular lies along and back of the 

 hyomandibular and symplectic bones ; the opercular is a broad 

 bone just back of the preopercular, the hinder border of which is 

 drawn out into a long spine ; the interopercular and subopercular 

 lie along the ventral borders of the preopercular and the opercular 

 respectively. 



Articulating with the ventral half of the hyoid arch are 

 seven slender arched bones, the branchiostegal rays, which sup- 

 port the branchiostegal membrane. 



Exercise 23. Draw a semidiagrammatic view of the side of the skull 

 on a scale of 2, and carefully label all the bones just mentioned. 



Disarticulate and remove the five gill arches and the hyoid arch 

 from the body. In the midventral area and joining these arches 

 is a row of median bones called the copulae ; the copula of the 

 hyoid arch is the basihyal, which supports the tongue and has 

 already been described. Note the gill rakers, the row of short 

 processes which spring from the inner surface of the gill arches ; 

 they act as strainers which permit water to pass through the gill 

 slits but keep solid objects out of them. Note gill rays, which 

 are on the outer surface of the arches and support the gills. 



