200 SUB-CLASS (AXD ORDER) TELEOSTEI. 



A pseudobranch * is generally present on the posterior side of 

 the hyoid arch. It contains a rete mirabile and usually has 

 the form of short filaments or ridges. In some cases it is con- 

 cealed below the mucous membrane, and the organ may have 

 the form of a red, lobed swelling (so-called glandular pseudo- 

 branch or vaso-ganglion). Sometimes it lies so far from the 

 surface that it is quite hidden : indeed it may be covered by 

 fat and muscles and even by bone. It is sometimes absent, 

 and its absence appears to be very generally correlated with 

 that of the choroid gland. The function of the pseudobranch 

 is unknown ; it lies in the course of the greater part of the blood 

 supply to the eye (see below), and it is generally regarded as a 

 vestige of a hyoid gill. 



Certain fishes, e.g. eels, can exist for some time out of water, 

 but those with large gill-apertures usually perish rapidly. In 

 some active fishes, e.g. the Scombridae, the temperature of the 

 blood is considerably higher than that of the water ; probably 

 it is always slightly higher, but we know very little on this 

 subject. 



The Teleosts have usually five gill-clefts, but the fifth is always smaller 

 than the rest and is sometimes absent. In this case the fourth branchial 

 arch bears one row of filaments only (demibranch) or may be gill-less. 

 In some forms the gill-apparatus, both arches and gills, may be still more 

 reduced (Symbranchidae, Malthe, etc. ; in Amphipnous cuchia the second 

 branchial arch alone bears gill-filaments). In some Lophobranchii the gills 

 have the form of curious tufted processes. 



Accessory respiratory structures are met with, especially in cases in 

 which the gill-filaments are reduced. Thus in Amphipnous there is a 

 lung-like vascular-lined sac, opening into the first gill-cleft, for breathing 

 air. In Saccobranchus there is a very similar sac. In Anabas scandens, 

 in which the gill-apparatus is complete, the superior pharyngeal bones are 

 honeycombed so as to form a laminated organ covered with vascular 

 mucous membrane to enable it to breathe out of water (Fig. 38). Accessory 

 respiratory organs are also found in the Ophiocephalidae, certain Siluridae 

 (Clarias, Heterobranchus, Heterotis], and in Chanos. 



The air-bladder is present in most but not in all Teleostei. 

 It presents great variety of structure throughout the group. 

 In the Malacopterygii, Ostariophysi, Apodes, and Haplomi it 

 usually, but not always, opens into the alimentary canal (usually 

 into the oesophagus) on its dorsal side by the pneumatic duct 

 (laterally in JErythrinus), which may however be partly or 



* Joh. Miiller, Vergl. Anat. der Myxinoiden, loc. cit. Maurer, Morph. 

 Jahrbuch, 9, 1883, p. 229. 



