In Protopterus[[295]] the number of branchial arches is increased to six, but, in consequence of the closure of the hyobranchial cleft, there are but five open clefts. The first, second, and third arches are wholly devoid of branchial filaments: the fourth and fifth support each a biserial gill, while the sixth arch retains only an anterior hemibranch, which, however, as the source of its blood supply seems to indicate, may consist of "emigrant" gill-filaments from the posterior hemibranch of the fifth arch.[[296]] Interbranchial septa are practically non-existent, the flattened, leaf-like gill-lamellae being free except at their attached bases, and thus repeating a characteristic Teleostean feature. A "hyoidean" hemibranch or pseudobranch, supplied from the ventral aorta, is present, but as the hyobranchial cleft is closed it projects into the branchial cavity immediately in front of the cleft between the first and second branchial arches. In Lepidosiren[[297]] the branchial arches are reduced to five and the clefts to four, the hyobranchial and fifth clefts being closed. There is a "hyoidean" hemibranch resembling that of Protopterus.
The facts furnished by the study of the numerical and structural variations in the gill-clefts, gills, and gill-arches of different groups of Fishes prove that atrophy of these structures takes place at opposite ends of the series. We have examples of this anteriorly in the suppression of the hyo-mandibular cleft and its hemibranch, and of the hyoidean hemibranch, as the result of the conversion of the mandibular and hyoid arches into jaws, or into skeletal supports for the jaws; and posteriorly, in the reduction which is evident when the generality of Fishes are compared with such primitive Elasmobranchs as Chlamydoselachus and Notidanus.
In most Fishes the concave pharyngeal margins of the branchial arches are fringed with a double series of either cartilaginous or bony tubercles or filaments, the "gill-rakers" (Figs. 161 and 164). The anterior row of gill-rakers on each arch usually interdigitate with those of the posterior row on the preceding arch, and in this way the two rows form a sieve-like mechanism to prevent any solid particles, which may enter the pharynx with the respiratory current of water, from passing into the gill clefts and clogging or otherwise injuring the branchial filaments.
In a few Fishes the gill-rakers are enormously developed, and subserve a function similar to that of the baleen plates of the Whalebone Whales in acting as a filter for straining from the water the small pelagic organisms on which the Fish feeds. This is notably the case in the great Basking Shark (Selache maxima)[[298]] in which the closely-set, flattened, tapering gill-rakers may be so long as four or five inches, and, while somewhat resembling "whalebone" in appearance, have the histological structure of vascular dentine. The nature of the food, which in the stomach of one specimen examined consisted solely of an immense quantity of plankton, including Copepods and the larvae of other Crustaceans,[[299]] affords clear evidence of the great value of such a filtering mechanism to this Shark, and, at the same time offers an explanation of the striking and significant reduction in the size of the teeth, which, relatively to the dimensions of the Fish, are so small as to be almost vestigial. A similar filter has been observed in an extinct Selache (S. aurata)[[300]] from the Antwerp Crag, and also in an existing South African Shark (Rhinodon typicus);[[301]] and in the latter, as in the Basking Shark, is associated with a marked reduction in the importance of the dentition. The long slender gill-rakers of the Chondrostean Polyodon also constitute an efficient filter, and the same may be said of several plankton-eating Teleosts.
The Mechanism of Respiration.—The aeration of the blood is effected by the rhythmical suction of water into the oral cavity, and its subsequent expulsion through the gill-clefts, bathing the highly vascular gill-lamellae in its course. In any single act of inspiration the mouth is opened, and the oral cavity enlarged by the lateral expansion of its walls. When the oral cavity is filled with water, the mouth is closed and the expiratory process begins. By the lateral contraction of the oral walls the water is driven outwards through the gill-clefts, and over the gill-lamellae. During this process the branchial arches become widely separated by the contraction of their muscles, the operculum is elevated, and the oesophagus is closed by the contraction of its muscular wall. In many Fishes the course of the expiratory water-current is controlled by special valve-like folds of the oral mucous membrane, the maxillary and mandibular "breathing-valves."[[302]]
The rate of "breathing" varies considerably in different Fishes, even in allied species.[[303]] In the Blue Wrasse (Labrus), and the Rockling (Motella), the number of respirations per minute is 15, in the Minnow (Leuciscus), and Stickleback (Gastrosteus), as many as 150. A deficiency of oxygen in the water accelerates the respiratory movements, and the Fish appears to "pant" or breathe hurriedly. In the Lampreys, both inspiration and expiration may take place through the external gill-apertures by the alternate expansion and contraction of the gill-sacs, more especially when the suctorial buccal funnel is used for the attachment of the animal. On the other hand, the singular habits of the Myxinoids involve a further modification of the respiratory process. In these Cyclostomata the inspiratory current enters the external naso-pituitary aperture and reaches the pharynx through the naso-pituitary canal, and thence, as an expiratory stream, traverses the gill-sacs on its way outwards. The pharynx is closed behind the last pair of gill-sacs by a constrictor muscle, which prevents the entrance of the water into the oesophagus, and converts the pharynx into a respiratory tube for the time being; but, when food is being swallowed, the pharyngeal constrictor is relaxed and the internal apertures of the gill-sacs are closed by the contraction of their own sphincter muscles.
In addition to the usual respiratory organs it is probable that in not a few Fishes the superficial skin may share with the gills the function of breathing. In this connexion may be mentioned the fact that in Periophthalmus the tail is used for respiration. Hickson[[304]] observed that a species of this genus, frequenting the extensive sandy shores of the Island of Celebes, often rests with its tail in the water, the head and trunk being exposed. Under such circumstances the gills are probably of little use, and the tail is utilised as a breathing organ, principally, as Haddon[[305]] subsequently pointed out, through the agency of its extremely vascular caudal fin.
Fig. 167.—Embryos of the Electric Torpedo (Torpedo ocellata). A, dorsal view; B, ventral view of a slightly younger specimen. cl, Cloaca; el.o, electric organ; ex.b, external gills; p.f, pectoral fin; pv.f, pelvic fin; sp, spiracle; y.s, stalk of yolk-sac.
Some Fishes possess larval breathing organs; others, even when provided with gills, either utilise the air-bladder, or develop special accessory organs, for aquatic or, more usually, for aerial respiration.