RESPIRATORY ORGANS. 241 
In the myxinoids the cesophago-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. 
Fic. 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 lamella 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 in Lepidosteus, 
most sturgeons and Ceratodus; they are lacking in Amia and Protopterus. Polyp 
terus and Polyodon have opercular gills. 
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