RESPIRATORY SYSTEM 759 



Polypterus), this cleft becomes smaller and smaller until there is only 

 a dorsal opening on the head the spiracle. In most vertebrates this 

 spiracle is closed in the adult but in the tailless amphibia and the higher 

 mammals the inner portion persists as the Eustachian tube and the 

 greater part of the middle ear. 



There are two types of gills in fishes. Practically all the elasmo- 

 branchs with the exception of the chimaeroids, have the interbranchial 

 septum well developed so that it extends beyond the demibranchs and 

 thus differentiates an excurrent canal in the cleft. The prolonged sep- 

 tum bends caudally at the outer end to protect the gills from injury. 



In teleostomes and chimaeroids the broad fold of the posterior end 

 of the hyoid arch grows backwards over the clefts to form a gill-cover 

 or opercular apparatus. The gill-cover encloses an extrabranchial or 

 atrial chamber into which the clefts empty. The chamber opens by a 

 single slit behind the operculum. 



In those instances, just mentioned, when an operculum is developed 

 the interbranchial septum is always reduced in size until there is only a 

 slender bar from which the demibranchs extend into the atrial cham- 

 ber. The two opercular folds are usually continuous beneath the 

 pharynx. 



In teleosts and ganoids the operculum (gill-cover proper) is usually 

 differentiated from a more ventral portion known as the branchiostegal 

 membrane which is quite flexible, and possesses a skeleton of slender 

 branchiostegal rays. The ventral wall of the pharynx in these cases is 

 nothing but a slender bar and is called the isthmus. 



Just as the air in the lungs in the higher forms of animals is taken 

 in through the outer air passages and then passes through the trachea, 

 bronchia, and bronchioles to the delicate septa in the lungs, so in animals 

 possessing gills there is likewise a delicate septum which separates the 

 blood from the stream of water which is constantly being passed over 

 the gills. Water is as a rule drawn into the mouth and as the enlarged 

 oral cavity contracts it is forced out through the clefts, passing over the 

 gills on its way. In the Myxinoids the oesophageo-cutaneous duct prob- 

 ably acts as the incurrent passage when the animal has the front of the 

 head immersed in the flesh of a fish. In the lampreys the water is prob- 

 ably taken and forced out through the gill clefts when the animal is 

 attached to some object. The spiracle serves as an incurrent opening 

 in many elasmobranchs, and it is provided with a valve which develops 

 from the anterior wall and closes to prevent any backflow. Sturgeons 

 and Polypterus have spiracles throughout life. 



Sharks have the gill clefts on the side in the so-called neck region 

 while skates have them on the lower surface of the body. This differ- 

 ence is brought about by the union of the anterior appendages with the 

 head in skates. 



Many teleosts have breathing valves at the mouth opening which 



