Teleosl The swim bladder of the teleost is a thin-walled, 

 midline sac in the dorsal mesentery. It lies below the kid- 

 neys: the gonads lie lateral to it and the gut is suspended 

 below it (Figure 9-13). The swim bladder may be connected 

 with the esophagus dorsally, or this connection may be 

 lacking in the adult (Figure 9-32). The salmon is an exam- 

 ple of the first, physostomous condition, while the perch is 

 an example of the second, physoclystous condition. This 

 bladder generally has a hydrostatic function, but in some 

 physostomes it is an air-respiratory organ. 



In both physostomes and physoclists the swim bladder 

 arises as a dorsal diverticulum from the esophagus. In the 

 cod the swim bladder wall contains peptic (stomach) glands. 

 In the physoclist the swim bladder undergoes modification 

 so as to function as a hydrostatic, sensory, or sound-produc- 

 ing organ. There is an anteroventral secretory region, the gas 

 gland, which contains a retia mirabilis (wonderful net) of 

 blood capillaries, and a posterodorsal absorbing region, the 

 oval, which develops from the degenerating pneumatic duct. 

 These two parts may lie in nearly separated anterior and 

 posterior chambers. The eel {Anguilla) is an intermediate 

 type, retaining the pneumatic duct while having the oval 

 and gas glands. 



In physostomes, blood is supplied to the swim bladder 

 through a branch of the coeliacomesenteric artery and is re- 

 turned to the heart through a vein joining the hepatic portal 

 system or the postcardinal vein {Anguilla). In physoclists the 

 coeliacomesenteric artery supplies the retia mirabilis of the 

 gas gland, the oval area, and the bladder wall. These struc- 

 tures also receive blood from the dorsal aorta. The blood 

 returns from the gas gland through the hepatic portal stem, 

 and the rest of the bladder is mainly drained through the 

 posterior cardinal veins. Right and left branches of the 

 vagus (Xth cranial) nerve innervate the structure, particu- 

 larly the gas gland; the oval is innervated by sympathetic 

 branches which parallel the vagus nerves. 



In some teleosts (Lampanyctus) the walls of the swim blad- 

 der are thickened with oil-filled cells and the lumen is 

 largely obliterated; however, a well-developed gas gland with 

 a complex vascular rete may be retained. A few teleosts lack 

 the swim bladder; these are usually specialized for lying on 

 the bottom. 



Am'ia, Lepisoiteus, Acipenser In Amia and Lepisosteus the 

 swim bladder extends nearly the length of the body cavity. 

 Unlike the teleost, the walls of the bladder are sacculated 

 laterally as in Neoceratodus. The pneumatic duct opens 

 through a dorsal, slit-like glottis in the region of the esophag- 

 eal constriction posterior to the pharynx. 



In Amia arterial blood comes from right and left pulmo- 

 nary arteries, while in Lepisosteus branches of the dorsal 

 aorta descend at intervals along the length of the bladder. 

 Venous drainage is into the left duct of Cuvier in Amia, into 

 the right postcardinal of Lepisosteus. The teleost Gymnarchus 

 is intermediate (Figure 9-33); the efferent branchial arteries 



of the third and fourth gill arches unite to form a root for 

 the coeliacomesenteric and pulmonary arteries. 



The swim bladder of the holostean arises as a single 

 evagination from the foregut, well back from the pharynx. 

 Bilateral anlagen have been described but only one is dis- 

 tinct, and this appears to be to the right of the midline. The 

 relative position of its attachment shifts forward with growth. 



In Acipenser the swim bladder is short and ovoid. The pneu- 

 matic duct enters the bladder ventrally, well behind the an- 

 terior end, and it joins the esophagus, or stomach, well 

 back from the pharynx. The point of entrance into the 

 esophagus is closed by simple constriction of the duct, and 

 there is no glottis. Internally the bladder is smooth walled. 

 The walls are fairly thick and fibrous — brittle in preserved 

 specimens. Isinglass, a very pure gelatin, is produced from 

 the swim bladders of sturgeons. The arterial and venous con- 

 nections are with the dorsal body channels. 



In the embryo, both left and right folds are said to arise 

 from the roof of the esophagus, but only the right gives rise 

 to the definitive swim bladder Bilateral anlagen have been 

 described for other fishes, and it has been assumed that some- 

 times the right and sometimes the left give rise to the swim 

 bladder. 



Polypterus Polypterus has bilateral air sacs joined anteriorly 

 and opening through a ventral glottis to the right of 

 the median line (Figure 9-14). It is assumed that these sacs 

 arise as bilateral diverticula from the underside of the 

 esophagus, just behind the pharynx. The left glottis is re- 

 duced to an epithelial strand or lost with the fusion of the 

 anterior ends of the sacs. The left sac is short, one-fourth of 

 the body cavity length; the right extends the entire body 

 length, passing up to the right of the esophagus and stomach 

 and back through the dorsal mesentery. In cross sections it 

 is seen to be broadly attached dorsally between the kidneys 

 with the gut suspended below it. 



The inner epithelium of the sacs is smooth and partly 

 ciliated. The walls are vascular and they are supplied dor- 

 sally by the pulmonary arteries and drained ventrally by pul- 

 monary veins which join to enter the sinus venosus. The 

 sacs are innervated by left and right vagi; the left has 

 a branch extending above the esophagus to the right sac. 

 Externally the walls of the sacs are covered by two layers 

 of striated muscle fibers. The outer fibers run posteriorly and 

 toward the ventral midline of each sac where they meet 

 along a raphe; the fibers in the deeper layer are oriented at 

 right angles to those of the outer. 



The sacs of Polypterus contrast strongly with those of other 

 actinopterygians (Figure 9-34). It is difficult to assume that 

 the chondrosteans, holosteans, and teleosts were derived from 

 a form with bilateral ventral sacs of which all trace has been 

 lost except in Polypterus. It is equally or more difficult to 

 assume that Polypterus is a modified choanate. The paired 

 sacs of Polypterus give foundation to the theory of the separa- 

 tion of a brachiopterygian group from the actinopterygian. 



284 • THE VISCERA 



