RESPIRATORY ORGANS. 



241 



In the myxinoids the oesophago-cutaneous duct is supposed to act as the 

 incurrent opening when these animals burrow into fishes. In the lam- 

 preys the water is said to pass both in and out through the gill clefts 

 when these animals are attached to some object. In at least some of 

 the elasmobranchs water passes in through the spiracle which regularly 

 opens and closes. 



Many, if not all of the teleosts have breathing valves. There are two pairs of 

 these, an anterior pair attached to the margins of the jaws, which permit the ingress 

 of the water but prevent its outflow. The other pair is formed by the branchiostegal 

 membrane, which closes the opercular opening and only allows -the water to pass 

 out. The action of both pairs can be easily seen from fig. 248. 



FIG. 248. Breathing valves of teleosts, after Dahlgren. A, schematic figure, the 

 anterior half in the vertical, the posterior in the horizontal plane; B, mouth of sunfish 

 (Eupomotis); b, branchiostegal valve; mn, mx, mandibular and maxillary valves; v, oral 

 valves. 



In certain fishes with an operculum (Acipenser, Lepidosteus, many teleosts) a 

 gill is developed as a series of lamellae on the inner surface of the operculum. This 

 opercular gill has respiratory functions. The pseudobranchs are homologous 

 with the true gills. They are developed in some elasmobranchs as vertical folds on 

 the anterior wall of the spiracular cleft, occurring in some cases, even where the 

 spiracle is closed externally. They, however, receive arterial blood and so cannot 

 be respiratory in function. The blood, still arterial in character, passes from them 

 to the chorioid coat of the eye and in some cases to the brain. From their position 

 they must be interpreted as the demibranch of the posterior side of the mandibular 

 arch. 



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

 hyomandibular bone. When free, they are gill-like in appearance, but in some 

 species (fig. 249) they are covered by muscles and connective tissue, when they 

 have a blood-red, glandular appearance. Pseudobranchs also occur inLepidosteus, 

 most sturgeons and Ceratodus; they are lacking in Amia and Protopterus. Polyp 

 terus and Polyodon have opercular gills. 

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