arches two, three, and four, and there are no hemibranchs. 



The branchial arches in this group have rudimentary gill 

 rakers on their inner margins, and the gill filaments extend 

 well beyond the narrow medial septa (Figure 11-17). No 

 gill rays support the filaments and there are two efferent 

 arteries in each arch, at least in Neoceralodus. 



The larvae of Protopterus and Lepidosiren have four pairs of 

 external gills early in life; these are replaced by internal 

 gills when the operculum develops (Figure 7-22). Neoceralo- 

 dus never has external gills, and the operculum appears 

 early. 



Acfinopferyg/an fishes The teleost type has a mandibular 

 hemibranch or pseudobranch, four holobranchs on the first 

 four arches, and sometimes a fifth arch hemibranch. A hyoi- 

 dean hemibranch is lacking. The branchial chamber is cov- 

 ered by the operculum laterally and opens posteriorly and 

 ventrally. The opercula may overlap ventrally or fuse; the 

 fusion may be extended to the isthmus, reducing the opercular 

 openings to bilateral slits or round openings. In Synbranchiis 

 the opercular opening is restricted to a ventral midline 

 fissure. 



The pseudobranch shows various stages of reduction and 

 covering. In the salmon it is well developed. In the perch 

 the filaments are reduced and covered by the mucous epithe- 

 lium of the pharynx, and this covering is carried to an ex- 

 treme in the cod where the hemibranch is buried. This 

 hemibranch is embedded in the roof of the pharynx of 

 Cyprtnus by closure of the pouch in which it lies. 



The mandibular hemibranch is limited in function with 

 the closing of the spiracle. Furthermore, it now receives 

 only oxygenated blood from the hyoid or first branchial 

 arch. The term pseudobranch refers to the lack of respira- 

 tory function of this hemibranch. The pseudobranch may 

 function in regulating the pressure in the ophthalmic artery 

 or serve an endocrine function; acidophile (acid staining) 

 cells are found in this organ of some fishes. 



In terms of its branchial structure, Eurypharynx represents 

 an extreme of the teleost — if indeed it is a teleost. In this 

 genus the branchial region has been moved far back from 

 the hyoid arch as a result of the development of the enor- 

 mous mouth gape. There are six branchial openings, the 

 first of which lies in front of the first branchial arch, as in- 

 dicated by the glossopharyngeal nerve passing behind it. 

 There are five holobranchs. The pouch-like external bran- 

 chial chambers, serving all of the gill slits of a side, open ven- 

 trally to either side of the swollen cardiac portion of the 

 isthmus. 



Teleosts show many respiratory modifications. Some have 

 arborescent or labyrinthine organs in the branchial cavity 

 which function as accessory air-breathing organs. In others 

 such as Saccobranchus, there are lung-like outgrowths of the 

 branchial cavity which extend back into the body muscula- 

 ture. In some, air is swallowed and oxygen absorbed through 

 the gut wall. 



The larvae of teleosts utilize the yolk sac or body surface 

 for respiration until the internal gills develop. Attenuated 

 filaments of the internal gills may extend well beyond 

 the opercular margin as external gills in types such as 

 Gymnarchus. 



Amia difi"ers from LepisosUus in lacking the hyoidean hemi- 

 branch (Figure 9-35) and in having the pseudobranch re- 

 duced and covered by the mucous epithelium. There are four 

 holobranchs in ,4mm just as in Lepisosteiis (Figure 1 1-19). In 

 Leptsosteus the hyoidean hemibranch ends in contact with 

 the posteroventral end of the mandibular pseudobranch. 

 Blood for the pseudobranch comes from the afferent hyoid 

 artery and the efferent artery of the first arch. The vessels 

 of the pseudobranch oi Amia interconnect the orbital and 

 ophthalmic stems; there is no direct efferent or afferent 

 branchial connection. Neither Amia nor Lepisosteiis have a 

 spiracle and the spiracular pouch is reduced to a rudiment, 

 a pore on the pharyngeal surface at the anterior medial end 

 of the interarcual groove. 



Acipenser is comparable to Lepisosteus except that the hyoi- 

 dean hemibranch liesjust inside the margin of the operculum 

 and does not contact the mandibular hemibranch (Figure 

 9-35). The spiracle is present but the spiracular pouch is 

 little more than a tube from the hyobranchial or anterior 

 branchial groove. Since the mandibular hemibranch re- 

 ceives only oxygenated blood, it is a pseudobranch. Acipen- 

 ser and Lepisosteus also agree in having the opercula joined 

 across the midline but free from the isthmus. 



In Scaphirhynchus the hyoidean hemibranch is reduced in 

 size and there is no spiracle. Polyodon lacks the hyoidean 

 hemibranch but is otherwise like Acipenser; it has a spiracle. 

 The larvae of these fishes are like those of the teleost. 



Polypterus has a wide spiracle. The inner surface of the 

 pharynx has the hyoidean or spiracular pouch separated 

 from the hyobranchial groove by a ridge of tissue. There are 

 four branchial openings. The first three branchial arches 

 have holobranchs and the last a hemibranch. There is no 



spiracle 



pseudobranch or 

 mandibular hemibranch 



ACIPENSER 



B 



LEPISOSTEUS 



Figure 9-35. Internal view of the operculum showing the relation- 

 ships of the hyoid hemibranch and the pseudobranch. 



288 • THE VISCERA 



