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 (citrium*), 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. (Palasichthyes) 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 atrium with a, Isivge si7ius veiiosns f 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 Avithout 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 lamellge 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 tei'mination 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 Avithin 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 pre\ent j^articles of solid matter 



* Latin for court or hall. 



t Latin for vein-cavity, or fold. 



