X TELEOSTEAN FISHES 353 



In a typical teleostean fish then the cartilaginous skeletonj con- 

 structed on the same plan as that of the Elasmobranch, becomes en- 

 sheathed and to a great extent replaced by bone. The extent to which 

 actual replacement takes place differs in different teleosts. Different 

 steps in the process are well seen in comparing the skull of a Salmon 

 with that of a Cod. In the Salmon the skull appears in external view 

 to be composed of a complicated array of bones, named for the most 

 part after bones that occupy roughly corresponding positions in Man 

 and other terrestrial vertebrates, but if these bones are removed there 

 is found underlying them a massive and well-developed cartilaginous 

 cranium, only small portions of which are replaced by bone. In the 

 Cod on the other hand while a similar array of investment bones are 

 seen on the surface of the skull, the cartilaginous cranium is found also 

 to be replaced so completely by bone that one would naturally describe 

 the Cod's skull as being entirely bony although as a matter of fact small 

 portions remain still cartilaginous. On the whole we may say that the 

 predominant characteristic of the adult teleostean skeleton as compared 

 with that of the Elasmobranch is that while constructed on the whole 

 of corresponding parts arranged on the sarne plan it is composed of bone 

 instead of cartilage. 



A conspicuous difference between the Teleost and the typical Elasmo- 

 branch is seen in the external opening of the gills, there being here in place 

 of the series of separate openings a single large opercular opening on each 

 side, bounded in front by a large gill-cover or operculum (Fig. 145, op). 

 The meaning of this difference is rendered apparent by making a 

 horizontal cut backwards from the angle of the mouth. It is then seen 

 (Fig. 148) that there are actually five separate gill-clefts opening out- 

 wards from the pharynx but that the outer portions of these clefts 

 have been thrown into one by the disappearance of the greater part of the 

 gill septum, the thickened pharyngeal edge of which alone persists. A 

 result of this is that the respiratory lamellae, instead of being attached 

 to the gill septum along almost their whole length, are attached only at 

 their inner ends, their outer portions hanging freely (Fig. 148, B, r.l, 

 and Fig. 149). While this change has come over the gill septa belonging 

 to the branchial arches, it does not take place in the case of the hyoid 

 arch. On the contrary the valvular flap formed by its outer portion, 

 which in the Elasmobranch covered over only the first branchial cleft 

 (Fig. 124, p. 299), is now greatly enlarged, covering in the whole series of 

 clefts behind the hyoid arch. This enlarged hyoidean valvular flap is 

 known as the operculum (Fig. 148, A, op). 



The Spiracle or first cleft, though it appears as a rudiment in the 



2 A 



