26o 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



branchiostegal membrane which closes the opercular opening and permits only 

 the water to flow out. The current is caused by the alternate enlargement and 

 constriction of the oral cavity. The action of the valves is easily understood 

 from figure 279. 



In some fishes with an operculum (Acipenser, Lepidosteus, Polyodon, Polyp- 

 terus many teleosts), an opercular gill with respiratory functions is developed as a 

 series of lamellae on the inner surface of the operculum. In some elasmobranchs, 

 even those in which the spiracle is closed, vertical folds (pseudobranchs) are 

 developed on the anterior wall of the cleft. These are homologous with gills, 

 and from their position must be regarded as the demibranch of the posterior 



Fig. 280. — Dissection of pseudobranchs (ps) and cephalic circle in pike (Esox), after 

 Maurer. cc, cephalic circle; e, vessels to eyes; g, gills; n, vessels to palate and nose; 

 l-IV, efiferent branchial arteries. 



side of the mandibular arch. On the other hand, they are not respiratory, as 

 they receive arterial blood, which passes from them to the chorioid coat of the 

 eye and in some cases to the brain. 



Pseudobranchs are common in teleosts, usually lying on the medial side of 

 the hyomandibular bone, but sometimes carried up to the floor of the skull. 

 When free they are gill-like in appearance, but in some species they are covered 

 by muscles and connective tissue (fig. 280), when they have a blood-red, 

 glandular appearance. They occur also in Lepidosteus, most sturgeons and in 

 Ceratodus, but none are found in Amia or Protopterus. 



In addition to what was said above regarding the amphibian gills, may be 



