Elementary Zoology, Second Edition

by Vernon L. Kellogg

Make a drawing of the sunfish from a lateral aspect, showing the external parts named.

Internal Structure.—Technical Note.—Insert one point of the scissors a little to one side of the anus and cut dorsally on the left side of the body to the backbone. Now cut anteriorly from the anus along the ventral wall to where the jaws unite, and cut, also anteriorly, along the dorsal wall until the left side of the body can be removed. Bend the opercular flap backward over the eye and pin the entire fish, uncut side down, to the bottom of the dissecting-pan, covering it with water.

The above operation will have severed the large powerful muscles forming the body-wall and extending along the sides. Note a membranous sac completely filling a large dorsal cavity. This is the swim-bladder, a float filled with air which tends to give the fish the same weight as the water it displaces. It arises as a diverticulum from the alimentary canal, but soon becomes permanently shut off from it. Beneath the swim-bladder is a large cavity filled with various organs, collectively known as the viscera. In vertebrate animals the cavity which contains the viscera is generally called the peritoneal cavity. It is lined by the peritoneum, a delicate membrane, part of which is deflected as the mesentery over the alimentary canal and the other organs, thus suspending them all from the dorsal wall. Note in the anterior end of the peritoneal cavity a large bi-lobed gland, the liver, red in fresh, yellowish in alcoholic specimens. Its function, like that of the liver of the toad, is to store up nutriment for the blood and to secrete a digestive fluid called bile. Behind the liver note a long, convoluted tube. What is this tube? Unfold this tube, separating it from its enveloping membrane, the mesentery. Thrust a probe down the throat and note that it passes into a thick-walled sac, the stomach. The mouth and gill-slits open into the front part of the alimentary canal called the pharynx, which leads by a short tube, the œsophagus, into the stomach. Note the large, thickened portion of the alimentary canal leading from the stomach. This is the pylorus, and to its walls are attached a number of finger-like projections, the pyloric cæca. The pyloric cæca secrete a fluid which is poured into the alimentary canal and which assists in the process of digestion somewhat as does the secretion from the pancreas of the toad. From the pylorus, passing backwards in one or two loops, is the small intestine. Trace this to its exit. Lying within the mesentery near the posterior end of the body-cavity note a small red glandular mass, the spleen.

At the anterior end of the body in front of the liver and between the sets of gills note the small pericardial cavity within which is contained the heart. The pericardial cavity is separated from the peritoneal cavity by a thick muscular wall against which the liver abuts. The heart consists of four parts. The posterior part is a thin-walled reservoir, the sinus venosus, into which blood enters through the jugular vein from the head and through the cardinal vein from the kidney. From the sinus venosus it passes forward into a large chamber, the auricle. Next it flows into the ventricle, where, by the contraction of the walls, rhythmical pulsations force it into the conus arteriosus, thence into the ventral aorta, and lastly into the gills, where it is purified. After passing through the capillaries in the fine gill-filaments it is again collected, now pure, by paired arteries from each pair of gills, which arteries unite to form the dorsal aorta extending backward just below the backbone to the end of the tail. From the dorsal aorta a pair of arteries, the subclavian, are given off to the pectoral fins. At this point two other arteries branch off ventrally, the first being the cardiac artery, which distributes blood to the stomach and pyloric cæca. The second divides into several long mesenteric arteries supplying blood to all parts of the intestine and spleen. In the caudal region blood is taken up through the caudal vein and carried forward to the kidneys. These strain out the impurities arising from waste of tissues, after which the blood is carried back to the sinus venosus through the cardinal vein. From the intestine it is gathered into the large portal vein as in the toad. The portal vein carries blood to the liver, where nutriment may be stored up, and from thence it flows back to the sinus venosus through a very short thin-walled vessel, the hepatic sinus.

The kidneys, more or less united in one mass, lie in the posterior part of the body-cavity along the dorsal wall. Note running from each side of the kidney a ureter which unites with its fellow and opens into a small urinary bladder which discharges through a small opening immediately back of the anus.

The reproductive organs lie below the swim-bladder near the posterior end of the body-cavity. If the fish are caught in the spring, the greater part of the body-cavity of the female is found to be filled with small eggs. When mature, these eggs are deposited by the mother fish in the gravel of the stream-bed where they are fertilized by the sperm-cells poured over them by the male and left floating in the water.

The nervous system of fishes is best studied in a specimen treated with nitric acid. Carefully remove the roof of the skull, thereby exposing the brain. Most anteriorly make out, as in the toad, the paired olfactory lobes. These are attached by long stalks to the cerebrum or forebrain, which is followed by two large hollow lobes, the midbrain or optic lobes. Behind the midbrain is the cerebellum. Following the cerebellum is the elongate medulla oblongata, which tapers backward into the spinal cord. How far backward does the spinal cord extend? On each side of the brain-case about opposite the cerebellum are located the auditory organs, each consisting of three semicircular canals which lie in different planes, and of the vestibule. These parts are filled with liquid, and suspended in the liquid in the vestibule are small calcareous bodies called otoliths or ear-stones. Running out beneath from the midbrain are the optic nerves, which cross, the left one connected with the right eye, the right one with the left eye. From each side of the medulla oblongata there is given off a large nerve, the vagus, which sends branches to the lateral line organs on either side, and extends backward to the stomach and viscera.

For further study of the nervous system see Parker's "Zootomy," pp. 122-130.

Make a drawing of the nervous system as worked out.