PISCES. 423 



short free filaments, e. g. Pleuronectidae, many Acanthopteri, or a vaso- 

 ganglion beneath the mucous membrane, e.g. Esox, Cyprinus, Gadus. The 

 remaining sets of gill-folds or filaments are borne upon the anterior and 

 posterior aspects of the septa or branchial arches, the branchial vein and 

 artery running at the base of each septum, or the outer aspect of the arch. 

 The gills are typically biserial. The fourth branchial arch, however, bears 

 only a single, the anterior, row of filaments in many Teleostei, e.g. Cyclop- 

 terus, many Plectognathi, or no gill at all, e.g. Lophius ; and in M althaea 

 among Pediculati (Acanthopteri} the third arch is uniserial, whilst the 

 second only is gill-bearing in Amphipnous cuchia (Physostomi Apodes). 

 The fifth arch has no gills except in Hexanchus and Heptanchus. Ceratodus 

 among Dipnoi, has four biserial gills, and the septa are well developed ; 

 Protopterus has biserial gills on the third and fourth branchial arches, and 

 only a single series on the fifth ' immigrants from the hind row of the fourth 

 arch,' seeing that the blood-supply is derived from the fourth branchial artery 

 (Boas). The gill-filaments of Protopterus are leaf-like. The Lophobranchii 

 among Teleostei are peculiar in having tufted processes in the place of gill- 

 filaments. 



The gill-filaments are prolonged externally in embryo Elasmobranchii, 

 in Acipenser, and many Teleostei, but are afterwards reduced. The young 

 Polypterus has two pairs of external gills supplied from the hyoidean 

 artery. Protopterus ', among Dipnoi, has three pairs of external gills, supplied 

 by the second, third, and fourth aortic arches, and attached close to the 

 summit of the shoulder-girdle. Accessory respiratory organs are found in 

 certain Teleostei. These take the form of (i) enlarged honey-combed 

 superior pharnygeal bones in Anabas scandens (Climbing Perch) and its 

 allies among Acanthopteri : (2) arborescent processes of the dorsal part of 

 the branchial arches in Clarias, Heterobranchus, Heterotis, among Siluroidei : 

 (3) a sac with vascular walls opening into the first gill-cleft in Amphipnous 

 cuchia and Saccobranchus singio (Siluroidei}. The above-named can exist 

 out of water, but this is not the case with Lutodeira chanos, a Clupeid, 

 which ' has an accessory gill developed in a curved caecal prolongation of 

 the branchial cavity' (Huxley). 



The outgrowth of the oesophagus, which forms the lungs of other 

 Vertebrata, is absent in Elasmobranchii^, Holocephali, some Teleostei, e.g. 

 Pleuronectidae, and various families and genera. Its aperture is ventral in 

 Polypterus and Dipnoi, dorsal elsewhere ; and in some Teleostei shifts back- 

 wards, even opening into the fundus of the stomach in Clupeidae. In this 

 family the Herring has a duct leading from the hind end of the air-bladder, 

 and opening on the left side of the sexual aperture. Protopterus has a 

 divaricator muscle to the aperture, which leads into a vestibule in it, 

 in Lepidosteus, Polypterus, and Lepidosiren. There is scarcely any duct in 



1 Unless represented by a dorsal caecum in some Sharks. 



