38 



aquatic JLitt 



water to pass out over the gills (u), 

 thus aeriating the blood passing through 

 them. From the pharynx a short oesoph- 

 agus (12) leads to the stomach (13), 

 which is a blind tube with the intestine 

 (15) leading out from it a little below 

 the centre. A short distance from the 

 stomach, along the intestine are several 

 pyloric coeca (14), which correspond to 

 some of the digestive glands in the higher 

 animals. These coeca empty digestive 

 fluids into the intestines. Below the be- 

 ginning of the intestine is the liver (18) 

 with its gall bladder (17) and a bile duct 

 (19) through which the bile flows into 

 the intestines. Another small organ 

 along the alimentary canal, but not con- 

 nected with it., is the spleen (16). The 

 function of this organ is still a mystery, 

 it being supposed, but not proven, that 

 it assists in the production of blood. In 

 passing it might be mentioned that herb- 

 ivorous species generally have a very 

 long intestine, which in some fish is coiled 

 around the air bladder, while in the car- 

 nivorous fishes the intestine is relatively 

 short. Finally the intestine ends at the 

 anal orifice (20), which is situated a 

 short distance in front of the anal fin. 



The air bladder (21), or swim bladder, 

 as it is sometimes called, is a compara- 

 tively large membranous sac, filled with 

 air, and generally reaching the length of 

 the body cavity. By means of muscular 

 movement it is contracted or expanded 

 accordingly as the fish wishes to sink or 

 rise. This contraction or expansion in- 

 creases or decreases the density of the 

 fish, thereby making it heavier or lighter 

 than water. Some fish that spend their 

 lives on the bottom have little or no swim 

 bladder, and are therefore unable to rise 

 or sink without the use of the fins. 



The gills (11) are eight in number, 

 four on each side. Each consists of a 

 bony arch, which supports the delicate 



gill filaments. The blood passing through 

 the fine capillaries in the gills gives off its 

 carbon dioxide from the body and takes 

 up a fresh supply of oxygen from the 

 water passing over the gills. Extending 

 into the pharynx from the gill arches are 

 a number of bones arranged like the 

 teeth in a comb these are called gill- 

 rakers and are supposed to act as strain- 

 ers. 



The heart is fairly simple, possessing 

 only one auricle (22) and one ventricle 

 (23) ; in the higher animals there are 

 two of each. It is placed in a rather 

 large cavity called the pericardial cavity, 

 the posterior or rear wall forming a thin 

 membrane between the pericardial and 

 the digestive or body cavity. The blood 

 coming from the body is received by the 

 auricle, which is a large, thin-walled 

 chamber, it then enters the ventricle, a 

 smaller, thick, muscular-walled chamber, 

 which it is pumped by a muscular 

 contraction through an artery called the 

 aorta (39) to the gills, where it is aeri- 

 ated. After aeriation the blood is collect- 

 ed by an artery called the dorsal artery, 

 through which it is distributed to the 

 various organs. In the fine capillaries 

 of the organs the blood gives up the sup- 

 ply of oxygen it is carrying and collects 

 carbon dioxide and other waste products. 

 It then flows back to the auricle of the 

 heart through the veins, thus completing 

 the circle. There is also a lymphatic 

 system, which acts in a supplementary 

 capacity to the blood. The fluid which 

 circulates in the lymphatic system is a 

 milky fluid called lymph, which helps to 

 distribute food throughout the system, 

 and also, to a certain extent, to pick up 

 waste matter. 



The principal organs of excretion are 

 the kidneys (24). These in the perch 

 are long, thin bodies, which extend along 

 the upper surface of the air bladder, 



