234 VERTEBRATE ANIMALS. 
heart, but is driven from the gills through all parts of the 
body, the propulsive 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 the circulation 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 circulation is effected by means 
of contractile dilatations situated upon several of the vessels. 
Inthe Mud-fishes (Zepzdoszven) 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 surrounding medium. The blood-corpuscles (fig. 
121, ¢) are always nucleated, and, except in the Lancelet, are 
all mostly red. . 
Whilst the respiration of all fishes is truly aquatic, most are, 
nevertheless, furnished with an organ which doubtless corre- 
sponds to (or is homologous with) the lungs of the higher Vertfe- 
évata. 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 by parti- 
tions. In most fresh-water fishes the gases in the swim-bladder 
are mainly composed of nitrogen, but in the sea-fishes it is 
chiefly filled with oxygen. The sac of the swim-bladder is 
often closed, but in other cases it opens into the gullet by means 
of a duct which corresponds to the windpipe. In the great 
majority of fishes the functions of the air-bladder are mainly 
hydrostatic—that is to say, it serves to maintain the necessary 
agreement between the specific gravity of the fish and that of 
the surrounding water. In the singular Mud-fishes (Lepzdo- 
stvyen), the air-bladder is composed of two distinct sacs, divided 
into a number of cellular compartments, and opening into the 
gullet by a tube. In this fish it acts as a respiratory organ, and 
_is therefore, not only in structure but also in function, the repre- 
sentative of the lungs of the other Vertebrates. 
The nervous system of fishes is of an inferior type of organisa- 
tion, the brain 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, though 
