FISHES. 235 



heart, but is driven from the gills through all parts of the body, the 

 pi-opulsive force necessary for this being derived partly from the 

 heart, and partly from the contractions of the muscles between 

 which the blood-vessels pass. The essential peculiarity of tlie circu- 

 lation of fishes consists in this, that the arterialised blood returned 

 from the gills is propelled through the general vessels of the body 

 (systemic vessels) without being sent back to the heart. In the 

 Lancelet, alone of all fishes, there is no single heart, and the circu- 

 lation is effected by means of contractile dilatations .situated upon 

 several of the vessels. In the Mud-fishes {Lepidosiren and Cera- 

 tudus) the heart consists of two auricles and a ventricle. In all 

 cases the blood is cold, or, in other words, has a temperature very 

 little, or not at all, higher than that of the suiroundiiig medium. 

 The red blood-corpuscles (fig. 163, c) are always nucleated, and are 

 oval in shape. 



Whilst the respiration of all fishes is truly aquatic, most are, 

 nevertheless, furnished with an organ which appai'ently corresponds 

 to (or is hoinologons with) the lungs of the higher Vertcbrata. This 

 is known as the "air-" or "swim-bladder," and is a sac filled with 

 gas and situated between the alimentary canal and the kidneys. In 

 most cases the sac contains only a single cavity, but in many 

 instances it is variously divided hy partitions. In most fresh-water 

 fishes the gases in the swim-bladder are mainly composed of nitro 

 gen, but in the .sea-fi.shes it is chiefly filled with oxygen. The .'iac 

 of the swim-bladder is often closed, but in other ca.ses it ojiens into 

 the gullet by means of a duct which corresponds to the windpiiie. 

 In the great majority of fishes the functions of the air-bladder are 

 mainly hydrostatic — that is to say, it serves to maintain the neces- 

 sary agreement between the specific gravity <jf the fish and that of 

 the surrounding water. In the singular Mud-fishes {LepidosircD 

 and Cvratodus), the air-bladder is composed of two distinct sacs, 

 divided into a number of cellular comp)artments, and opening into 

 the gullet by a tube. In these fishes it acts as a respiratory organ, 

 and is therefore, not only in structure, but also in function, the 

 representative of the lungs of the other Vei-tebrates. 



The nerv(jus system of fi.shes is of an inferior type of organisa- 

 tion, tlie l)rain being of comparatively small .size, and consisting of 

 a collection of ganglia. As regards the organs of the senses, two 

 peculiarities deserve notice. In the first place, tho\igh fishes possess 

 the essential parts of the organ of hearing, they jiossess no external 

 ears, and mostly no external auditory openings. In the second 

 place, the organ of smell consists of a double cavity lined by a 

 mucous niendirane folded into numerous plaits, into which water 

 is admitted, usually by two distinct apertures or nostrils. Behind, 



