4, NEW SOUTH WALES 
which send the blood on from the veins direct to the gills, Thus all 
the veins ultimately discharge their contents into a large chamber of the 
heart (atriwm*), whence the blood passes by the pulsation of the ventricle 
into the cone or auricle. Valves here prevent its return. The cone or 
bulb of the auricle is prolonged into the gill artery, where it soon divides, 
sending off a branch to each of the gill arches. After being well revived 
by the oxygen of the water, the blood returns from the gills, some to 
arteries where a portion is sent off to different parts of the head and 
heart, but the main trunk unites to form a great artery which carries 
blood to all parts of the muscles, intestines, and tail. In the great 
majority of fishes it is only the ventricle which contracts. In sharks, 
rays, &c. (Paleichthyes) the auricle also beats or pulsates, It is in- 
teresting to study the hearts of fishes, from which much can be learned. 
Its simple structure is one that can be easily understood. In the 
anterior part will be found the atriwm with a large sinus venosus} or 
groove, into which all the veins enter, then the ventricle, and then a 
conical hollow swelling at the beginning of the arterial system. In all 
the sharks and rays this swelling is still a division of the pulsating 
heart. It has a thick muscular layer, but is not separated from the 
ventricle by two valves (like the heart of warm-blooded animals) opposite 
to each other, yet its interior is provided with many valves arranged in 
a transverse series. In Cyclostomata and Teleosteans the enlargement is 
a swelling of the artery without a muscular thickening, and it does not 
pulsate. 
The reference made to the crossing of the optic nerves is worthy of 
particular notice. The optic nerves of fishes take their rise from the 
optic lobes of the brain. In the Cyclostomata they go straight to the 
eye each on its own side. In the Teleosteans they simply cross each 
other so that each optic nerve supplies the eye on the other side of the 
brain. In sharks and rays the optic nerves unite after leaving the 
brain and become merged into one. This compound nerve is cylindrical 
for a time, but soon flattens out like a plaited band which can be 
separated and expanded. 
The blood, as already explained, is carried to the gills. These are the 
lungs of fishes, only lungs are organs especially designed to breathe 
air, while gills breathe water. Both, however, are for the same purpose, 
that is, renewing the blood by the oxygen which both air and water con- 
tain, while at the same time it parts with certain elements which it has 
removed from the system. In gills this is effected between what are 
called the branchial arches. They are usually five in number, of which 
four bear gills, while the fifth remains dwarfed. 
The gills are fine lamelle covered with mucous membrane, every one 
of which is supplied with a small twig from the branchial artery. This 
is subdivided into the finest branchlets, and their termination being very 
near the surface is what gives the red colour to the gills. The water 
washing over these surfaces while the blood is moved along by the heart, 
brings every portion in succession within reach of the oxygen which is 
rapidly absorbed. On the inner side of the gills there are sets of organs 
called gill-rakers, which act as a sieve to prevent particles of solid matter 
* Latin for court or hall, 
{ Latin for vein-cavity, or fold. 
