CHARACTERS OF FISHES. 349 



below the spine. Even where the vertebral column is not pro- 

 longed into the upper lobe, the tail may nevertheless become 

 heterocercal, in consequence of a great development of the 

 haemal spines as compared with the neural spines of the 

 vertebrae. 



The process of respiration in all fishes is essentially aquatic, 

 and is carried on by means of branchial plates or tufts devel- 

 oped upon the posterior visceral arches, which are persistent, 

 and do not disappear at the close of embryonic life, as they do 

 in other Vertebrates. In the Lancelet alone, respiration is 

 effected partly by branchial filaments placed round the com- 

 mencement of the pharynx, and partly by the pharynx itself, 

 which is greatly enlarged, and has its walls perforated by a 

 series of transverse ciliated fissures. The arrangement and 

 structure of the branchiae differs a good deal in the different 

 orders of Fishes, and these modifications will be noticed sub- 

 sequently. In the meanwhile it will be sufficient to give a 

 brief description of the branchial apparatus in one of the bony 

 fishes. In such a fish, the branchiae are connected with the 

 hyoid arch, and are situated in two special chambers, situated 

 one on each side of the neck. The branchiae are carried 

 upon the outer convex sides of what have been already described 

 as the "branchial arches ;" that is to say, upon a series of bony 

 arches which are connected with the hyoid arch inferiorly, and 

 are united above with the base of the skull. The internal 

 concave sides of the branchial arches are usually furnished 

 with a series of processes, constituting a kind of fringe, the 

 function of which is to prevent foreign substances finding their 

 way amongst the branchiae, and thus interfering with the proper 

 action of the respiratory organs. The branchiae, themselves, 

 usually have the form of a double series of cartilaginous leaflets 

 or laminae. The branchial laminae are flat, elongated, and 

 pointed in shape, and they are covered with a highly vascular 

 mucous membrane, in which the branchial capillaries ramify. 

 The blood circulates through the branchial laminae, and is here 

 subjected to the action of aerated water, whereby it is oxygen- 

 ated. The water is constantly taken in at the mouth by a 

 movement analogous to swallowing, and it gains admission to 

 the branchial chambers by means of a series of clefts or slits, 

 the " branchial fissures," which are situated on both sides of the 

 pharynx. Having passed over the gills, the deoxygenated 

 water makes its escape posteriorly by an aperture called the 

 " gill-slit " or " opercular aperture," one of which is situated on 

 each side of the neck. As we have seen before, the gill-slit is 

 closed in front by a chain of flat bones, collectively consti- 



