VISCERAL SKELETON. 



191 



epihyal is joined to the cartilage interval in the hyomandibular 

 element between the hyomandibular and the symplectic 

 bones by a small osseous piece the interhyal {d), while ventrally 

 the hypohyal joins the large median, sometimes toothed 

 glossohyal. In connection with the hyoid arch is a number of 

 membrane bones — the opercular bones supporting the opercu- 

 lum, and the branchiostegal rays in the branchiostegal mem- 

 brane. The opercular bones are in connection with the dorsal, 

 hyomandibular element and consist of the operculum (Fig. 110, 

 Op) and preoperculum {POp), and sometimes of a suboperculum 



Fig. 111. Hyoid apparatus and branchial arches of Perca fluviatilis (Eegne animal). 

 a, b c, d segments of the branchial arches ; the upper joints Ops are the superior 

 pharyngeal bones (pharyngobranchials) ; VI, Opi the inferior pharyngeal bones (reduced 

 fifth branchials) ; Cop median pieces (copulae) ; Rb branchiostegal rays ; I=Zbg 

 hyoid arch ; // — F branchial arches. 



(SOp) and interoperculum {J Op). The branchiostegal rays 

 {Brs) are attached to the lower part of the hyoid arch, partly to 

 the inner and partly to the outer side (Fig. Ill Eb). There are 

 five pairs of branchial arches. Of these the anterior four are 

 usually segmented into four pieces (Fig 111), the pharyngo- 

 {Ops=:d), epi-(c), cerato-(6) and hypo-(o) branchials. More or 

 fewer of the pharyngo-branchials, which are not joined to the 

 skull or vertebral column, are united with one another to form 

 the so-called superior pharyngeal hone which generally bears 



