374 Vertebrata. 



rods. The septa between the gill clefts have become narrower, espe- 

 cially in the Teleostei, whilst in the Selachians they are broad plates, 

 not completely covered by gill lamellae, so that there is a free edge 

 externally; this is absent from the group just mentioned, and the 

 outer ends of the gill-lamellEe project to a certain extent over the 

 outer margin of the septum : this is the case in the Teleosteans, 

 where the narrow, pointed laminse usually arise by a short basal 

 portion from the much reduced septum. In these groups the two 

 series of laminae belonging to one gill-bar constitute a gill, of 

 which four pairs are present ; the cavity within the operculum, into 

 which the gills project, is called the branchial chamber. In 

 Ganoids, and Dipnoans, a series of gill-lamellae is often still present on 

 the posterior side of the hyoid, within the operculum, the opercular 

 gill; whilst it is rudimentary, or wanting in the Teleostei : the 

 spiracle is also retained in the Sturgeon and Polypterus. 



The operculum is attacied to the hyoid, and contains flat and rod-like 

 niem.brane bones (Fig. 320). In the Teleostei, along the posterior edge 

 of the upper portion of the hyoid, is a long bone, the preoperculnm, 

 behind this are three large flat ones, operculum, sub- and inter- 

 operculum; and from the lower portion of the hyoid, arises a series of 

 thin curved pieces, the branohiostegal rays, embedded in the lower 

 membranous portion of the operculum. The external opening of the branchial 

 chamber is usually a large slit, in some Fish {e.g., the Bel), however, the hinder 

 boi'der of the operculum is eonoresoent with the body to such a large extent, that 

 only a small lateral aperture remains. 



In most Pisces, water enters the buccal-cavity through the 

 mouth, which is then closed, whilst the tongue is raised, and the 

 operculum pressed in, so that the water is driven through the gill- 

 slits over the gill lamellae. In the Selachians, the water is 

 taken in, not by the mouth, but by the spiracle. In the Cyclo- 

 s tomes, it is generally both received and ejected by the external 

 branchial aperture. At the inner edge of the gill-bars is a more or 

 less well-developed straining apparatus, the gill-rakers, 

 whose function is to prevent the solid bodies, which enter the 

 oral-cavity with the water, from passing into the gill-sacs or 

 branchial chamber. In the Selachians, the Dipnoans, aiid the 

 cartilaginous Ganoids, this apparatus usually consists of a double 

 series (single on the hyoid and the last gill-arch) of cartilaginous 

 rods, on the inner edge of the gill-arch; the rods of the anterior 

 rows on each arch dovetail with those of the posterior row on the 

 preceding bar. In the Teleostei they are often replaced by bony 

 outgrowths, which may be dentigerous ; the anterior series of the 

 first gill-arch is often composed of very long rods, projecting over 

 the cleft between the first arch and the hyoid, upon which they do 

 not occur. For the rest, they are developed to very different extents 

 in different Teleosteans, in some, e.g., the Herring, very well deve- 

 loped ; in others, quite insignificant. 



