372
K JELL JOHANSEN
Chondrostei (Polypterus) and Holostei ( Amia and Lepisosteus) have
also retained a respiratory function of the air bladder but fall short of
the refinement in structural adjustment for air breathing seen in the dipnoans (Klika and Lelek, 1967). Among modem teleosts some have
secondarily developed the air bladder into a respiratory organ such as
seen in the South American fishes Erythrinus uniteniatus and Arapaim
gigus (Carter and Beadle, 1931b; Sawaya, 1946).
Only in the lungfishes, and of these Protopterus and Lepidosiren
in particular, has the air bladder evolved structurally to resemble a lung
as it exists in lower vertebrates. Internal septa, ridges, and pillars divide
the air space into smaller compartments opening into a median cavity.
Further subdivisions of these compartments b y progressively smaller
reticulated septa terminates in alveoli-like pockets richly covered with
blood vessels. Neoceratodus has one lung, while Protopterus and Lepidosiren have two lungs fused in their anterior region where the pneumatic
duct and the pneumatic sphincter connect the lungs with the esophagus.
The short airway duct, which in no way resembles the air distributing
system of the trachea and bronchii in terrestrial vertebrates, is richly
supplied with smooth muscle as is the lung parenchyma itself. The
smooth musculature is important for the mechanics of breathing and
may exert additional influence in distributing the air inside the lung
(Grigg, 1965; Johansen and Reite, 1967). Electron microscopy has revealed that the fine structure of the lung in Protopterms does not differ
basically from that in higher vertebrates (DeGroodt et al., 1960;
Schulz, 1960). Klika and Lelek (1967), reporting on lung structure in
Protopterus, state that the respiratory epithelium adjoins the basement
membrane which in turn is separated from the endothelial basement
membrane by a thin layer of collagen fibrils. The approximate blood
to air diffusion distance in Protopterms is about 0.50 p . The Protopterus
lung is much more similar to the amphibian lung than that of Polypterus
which has a smooth inner surface much less vascular than in the lungfishes. Figure 3A shows the gross structure of the Protopterus lung.
Structural specializations in the perfusion pattern through the gas
exchange organs and the vascular linkage between them and the general circulation is of utmost importance for efficient gas exchange. In airbreathing fishes presumably the highest degree of efficiency will prevail when the air-breathing organ is perfused with systemic venous
blood and when the respiratory efferent blood is dispatched directly
for systemic arterial distribution. Most air-breathing fishes also employ
aquatic breathing and efficient perfusion of the gills and/or skin is
important since these areas are the primary sites for CO, elimination.
The vascular system should hence ensure that both the air- and waterbreathing respiratory organs are perfused before the blood is turned
over to the nutritive circulation.
K JELL JOHANSEN
Chondrostei (Polypterus) and Holostei ( Amia and Lepisosteus) have
also retained a respiratory function of the air bladder but fall short of
the refinement in structural adjustment for air breathing seen in the dipnoans (Klika and Lelek, 1967). Among modem teleosts some have
secondarily developed the air bladder into a respiratory organ such as
seen in the South American fishes Erythrinus uniteniatus and Arapaim
gigus (Carter and Beadle, 1931b; Sawaya, 1946).
Only in the lungfishes, and of these Protopterus and Lepidosiren
in particular, has the air bladder evolved structurally to resemble a lung
as it exists in lower vertebrates. Internal septa, ridges, and pillars divide
the air space into smaller compartments opening into a median cavity.
Further subdivisions of these compartments b y progressively smaller
reticulated septa terminates in alveoli-like pockets richly covered with
blood vessels. Neoceratodus has one lung, while Protopterus and Lepidosiren have two lungs fused in their anterior region where the pneumatic
duct and the pneumatic sphincter connect the lungs with the esophagus.
The short airway duct, which in no way resembles the air distributing
system of the trachea and bronchii in terrestrial vertebrates, is richly
supplied with smooth muscle as is the lung parenchyma itself. The
smooth musculature is important for the mechanics of breathing and
may exert additional influence in distributing the air inside the lung
(Grigg, 1965; Johansen and Reite, 1967). Electron microscopy has revealed that the fine structure of the lung in Protopterms does not differ
basically from that in higher vertebrates (DeGroodt et al., 1960;
Schulz, 1960). Klika and Lelek (1967), reporting on lung structure in
Protopterus, state that the respiratory epithelium adjoins the basement
membrane which in turn is separated from the endothelial basement
membrane by a thin layer of collagen fibrils. The approximate blood
to air diffusion distance in Protopterms is about 0.50 p . The Protopterus
lung is much more similar to the amphibian lung than that of Polypterus
which has a smooth inner surface much less vascular than in the lungfishes. Figure 3A shows the gross structure of the Protopterus lung.
Structural specializations in the perfusion pattern through the gas
exchange organs and the vascular linkage between them and the general circulation is of utmost importance for efficient gas exchange. In airbreathing fishes presumably the highest degree of efficiency will prevail when the air-breathing organ is perfused with systemic venous
blood and when the respiratory efferent blood is dispatched directly
for systemic arterial distribution. Most air-breathing fishes also employ
aquatic breathing and efficient perfusion of the gills and/or skin is
important since these areas are the primary sites for CO, elimination.
The vascular system should hence ensure that both the air- and waterbreathing respiratory organs are perfused before the blood is turned
over to the nutritive circulation.