Traces of what appear to be pre-spiracular clefts exist in the embryos of various forms. Perhaps the most remarkable of these is to be found in the larval Crossopterygian,[12] and apparently also in Amia[13] at least, amongst the other ganoids, where a pair of entodermal pouches become cut off from the main entoderm and, establishing an opening to the exterior, give rise to the lining of the cement organs of the larva. Posteriorily there is evidence that the extension backwards of the series of gill clefts was much greater in the primitive fishes. In the surviving sharks (Chlamydoselachus and Notidanus cinereus), there still exist in the adult respectively six and seven branchial clefts, while in embryonic Selachians there are frequently to be seen pouch-like outgrowths of entoderm apparently representing rudimentary gill pouches but which never develop. Further evidence of the progressive reduction in the series of clefts is seen in the reduction of their functional activity at the two ends of the series. The spiracle, even where persisting in the adult, has lost its gill lamellae either entirely or excepting a few vestigial lamellae forming a “pseudobranch” on its anterior wall (Selachians, sturgeons). A similar reduction affects the lamellae on the anterior wall of the hyobranchial cleft (except in Selachians) and on the posterior wall of the last branchial cleft.
A pseudobranch is frequently present in Teleostomes on the anterior wall of the hyobranchial cleft, i.e. on the inner or posterior face of the operculum. It is believed by some morphologists to belong really to the cleft in front.[14]
Phylogeny.—The phylogeny of the gill clefts or pouches is uncertain. The only organs of vertebrates comparable with them morphologically are the enterocoelic pouches of the entoderm which give rise to the mesoderm. It is possible that the respiratory significance of the wall of the gill cleft has been secondarily acquired. This is indicated by the fact that they appear in some cases to be lined by an ingrowth of ectoderm. This suggests that there may have been a spreading inwards of respiratory surface from the external gills. It is conceivable that before their walls became directly respiratory the gill clefts served for the pumping of fresh water over the external gills at the bases of which they lie.
| Fig. 9.—Lung of Neoceratodus, opened in its lower half to show its cellular pouches. a, Right half; b, Left half; c, Cellular pouches; e, Pulmonary vein; f, Arterial blood-vessel; oe, Oesophagus, opened to show glottis (gl.) |
Lung.—As in the higher vertebrates, there develops in all the main groups of gnathostomatous fishes, except the Selachians, an outgrowth of the pharyngeal wall intimately associated with gaseous interchange. In the Crossopterygians and Dipnoans this pharyngeal outgrowth agrees exactly in its mid-ventral origin and in its blood-supply with the lungs of the higher vertebrates, and there can be no question about its being morphologically the same structure as it is also in function.
In the Crossopterygian the ventrally placed slit-like glottis leads into a common chamber produced anteriorly into two horns and continued backwards into two “lungs.” These are smooth, thin-walled, saccular structures, the right one small, the left very large and extending to the hind end of the splanchnocoele. In the Dipnoans the lung has taken a dorsal position close under the vertebral column and above the splanchnocoele. Its walls are sacculated, almost spongy in Lepidosiren and Protopterus, so as to give increase to the respiratory surface. In Nexeratodus (fig. 9) an indication of division into two halves is seen in the presence of two prominent longitudinal ridges, one dorsal and one ventral. In Lepidosiren and Protopterus the organ is completely divided except at its anterior end into a right and a left lung. The anterior portion of the lung or lungs is connected with the median ventral glottis by a short wide vestibule which lies on the right side of the oesophagus.
In the Teleostei the representative of the lung, here termed the swimbladder, has for its predominant function a hydrostatic one; it acts as a float. It arises as a diverticulum of the gut-wall which may retain a tubular connexion with the gut (physostomatous condition) or may in the adult completely lose such connexion (physoclistic). It shows two conspicuous differences from the lung of other forms: (1) it arises in the young fish as a dorsal instead of as a ventral diverticulum, and (2) it derives its blood-supply not from the sixth aortic arch but from branches of the dorsal aorta.
These differences are held by many to be sufficient to invalidate the homologizing of the swimbladder with the lung. The following facts, however, appear to do away with the force of such a contention. (1) In the Dipneusti (e.g. Neoceratodus) the lung apparatus has acquired a dorsal position, but its connexion with the mid-ventral glottis is asymmetrical, passing round the right side of the gut. Were the predominant function of the lung in such a form to become hydrostatic we might expect the course of evolution to lead to a shifting of the glottis dorsalwards so as to bring it nearer to the definitive situation of the lung. (2) In Erythrinus and other Characinids the glottis is not mid-ventral but decidedly lateral in position, suggesting either a retention of, or a return to, ancestral stages in the dorsalward migration of the glottis. (3) The blood-supply of the Teleostean swimbladder is from branches of the dorsal aorta, which may be distributed over a long anteroposterior extent of that vessel. Embryology, however, shows that the swimbladder arises as a localized diverticulum. It follows that the blood-supply from a long stretch of the aorta can hardly be primitive. We should rather expect the primitive blood-supply to be from the main arteries of the pharyngeal wall, i.e. from the hinder aortic arch as is the case with the lungs of other forms. Now in Amia at least we actually find such a blood-supply, there being here a pulmonary artery corresponding with that in lung-possessing forms. Taking these points into consideration there seems no valid reason for doubting that in lung and swimbladder we are dealing with the same morphological structure.
Function.—In the Crossopterygians and Dipnoans the lung is used for respiration, while at the same time fulfilling a hydrostatic function. Amongst the Actinopterygians a few forms still use it for respiration, but its main function is that of a float. In connexion with this function there exists an interesting compensatory mechanism whereby the amount of gas in the swimbladder may be diminished (by absorption), or, on the other hand, increased, so as to counteract alterations in specific gravity produced, e.g. by change of pressure with change of depth. This mechanism is specially developed in physoclistic forms, where there occur certain glandular patches (“red glands”) in the lining epithelium of the swimbladder richly stuffed with capillary blood-vessels and serving apparently to secrete gas into the swimbladder. That the gas in the swimbladder is produced by some vital process, such as secretion, is already indicated by its composition, as it may contain nearly 90% of oxygen in deep-sea forms or a similar proportion of nitrogen in fishes from deep lakes, i.e. its composition is quite different from what it would be were it accumulated within the swimbladder by mere ordinary diffusion processes. Further, the formation of gas is shown by experiment to be controlled by branches of the vagus and sympathetic nerves in an exactly similar fashion to the secretion of saliva in a salivary gland. (See below for relations of swimbladder to ear).
Of the important non-respiratory derivatives of the pharyngeal wall (thyroid, thymus, postbranchial bodies, &c.), only the thyroid calls for special mention, as important clues to its evolutionary history are afforded by the lampreys. In the larval lamprey the thyroid develops as a longitudinal groove on the pharyngeal floor. From the anterior end of this groove there pass a pair of peripharyngeal ciliated tracts to the dorsal side of the pharynx where they pass backwards to the hind end of the pharynx. Morphologically the whole apparatus corresponds closely with the endostyle and peripharyngeal and dorsal ciliated tracts of the pharynx of Amphioxus. The correspondence extends to function, as the open thyroid groove secretes a sticky mucus which passes into the pharyngeal cavity for the entanglement of food particles exactly as in Amphioxus. Later on the thyroid groove becomes shut off from the pharynx; its secretion now accumulates in the lumina of its interior and it functions as a ductless gland as in the Gnathostomata. The only conceivable explanation of this developmental history of the thyroid in the lamprey is that it is a repetition of phylogenetic history.