184 SUB-CLASS (AND ttKDEH) TKLF.osTEI 



trunk, and the vent that Vjetween the trunk and tail. The mouth 

 is usually at the anterior end of the head,but it varies considerably 

 in form and position ; it may be anterior or superior (on the upper 

 surface), or inferior or lateral (extending along each side of the 

 head). The upper jaw may be formed by the premaxillaries 

 (intermaxillaries) and maxillaries or by the premaxillaries only, 

 and both jaws may be provided with tactile appendages called 

 barbels. The nostrils are usually double on each side, and in a few 

 cases (some eels) the anterior or lower opening perforates the 

 upper lip. The eyes are large, are usually without lids, and the 

 cornea is flat. In cave-fishes and in abyssal forms they may be 

 much reduced and even hidden beneath the skin, but in some 

 deep-sea forms they are enormously enlarged. The branchial 

 slits are hidden by the gill-cover, which is a fold of skin, con- 

 taining dorsally four flat bones, the opercular bones or opercles, 

 and ventrally a number of bony rods called the branchiostegal 

 rays. The branchial aperture or aperture of the space bounded 

 by the operculum is usually a slit of considerable dorso-ventral 

 extent, but in some cases it is much restricted, and in Sym- 

 branchus, the openings of the two sides coalesce in the middle 

 line. The space on the throat between the gill apertures is called 

 the isthmus. There is no spiracle. The actual gill-openings 

 themselves, i.e. the openings leading from the pharynx into the 

 space below the gill-cover are not tubular but simply slits, 

 separated by the rod-like branchial arches. These carry the gills 

 which are usually filiform and project freely. There are usually 

 two rows of them (holobrancks) on each branchial arch, projecting 

 like the prongs of a comb ; hence they are said to be pectinate 

 (Fig, 118). There are usually five gill-slits, but the last is always 

 smaller than the others, and is sometimes absent, in which case 

 the fourth branchial arch bears only one row of gill-filaments 

 (hemibranch). They may be still further reduced. There are 

 five branchial arches, but the last never bears filaments. 

 The hyoid never carries true gill-filaments, but it generally 

 carries them in a reduced form as a pseudobranch. In some 

 cases the pseudobranch is without a trace of filaments and has 

 the form of a red glandular patch. The vent is usually at the 

 junction of the trunk and tail, but it may be shifted far back on 

 the tail or far forward on the trunk. It consists of at least two 

 openings placed close together, the foremost being the anus 



