CHARACTERS OF FISHES. 395 



the form of the tail fishes exhibit two very distinct types of 

 structure, termed respectively the "homocercaP' and " hetero- 

 cercal" type of tail (fig. 167). The homocercal tail is the one 

 which most commonly occurs in our modern fishes, and it is 

 characterised by the fact that the two lobes of the tail are 

 equal, and the vertebral column, instead of being prolonged 

 into the upper lobe of the tail, stops short at its base. In 

 the heterocercal tail, on the other hand, the vertebral column 

 is prolonged into the upper lobe of the tail, so that the tail 

 becomes unequally lobed, its greater portion being placed 

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

 longed into the upper lobe, the tail may nevertheless become he- 

 terocercal, 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 deve- 

 loped upon the posterior visceral'arches, which "are "p'erststentT 

 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 differ 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 ofcaHiTagi- 

 nous leaflets or laminae. The branchial laminae are flat, elon- 

 gated, 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 



