The foregoing dissection will expose the central nervous system. It extends as a series of paired ganglia connected by a double nerve-cord along the ventral median line from the œsophagus to the last segment of the abdomen. From what points do the lateral nerves arise? Anteriorly the double nerve-cord divides, the two parts passing upward on each side of the œsophagus, where they again meet to form the supra-œsophageal ganglion or brain. Where do the nerves run which rise from the brain? What is the difference between the position of the central nervous system in the crayfish and in the toad?
Make a drawing of the nervous system.
Just beneath the nerve-cord note a blood-vessel extending the length of the body. This is the sternal artery, which arises from the posterior end of the heart and passes ventrally at one side of the alimentary canal and between the nerve-cords. Here the sternal artery divides into an anterior and a posterior branch, from which lesser branches are given off to each one of the appendages. The various arteries running to all parts of the body finally pour out the blood into the body-cavity, where it flows freely in the spaces among the various tissues and organs. After the blood has bathed the body tissues it flows to the gills on either side, passing up the outer side of the gill through delicate thin-walled vessels, where it is oxygenated as has already been described. From the gills the purified blood flows back on the inner side through a large chamber, sinus, into the pericardium, through the ostia of the heart, whence it is driven into the arteries once more. This sort of a circulatory system in which the blood in places is not enclosed in a definite vessel is known as an open system. In the toad we find the blood in a closed system, i.e., arteries leading into capillaries which in turn lead into veins, in no case allowing the blood to pass freely through the spaces of the body.
[CHAPTER V]
THE MODIFICATION OF ORGANS AND FUNCTIONS
Differences between crayfish and toad.—In the dissection of the crayfish one of the most important things in the study of zoology has been learned. It is plain that the crayfish has a body composed, like the toad's, of parts or organs, and that most of these organs, although differing much in appearance and actual structure from those of the toad, correspond to similarly named organs of the toad, and perform the same functions or processes, although with many striking differences, essentially in the same way as in the toad. But the structure of the body is very different in the two animals. The toad has an internal body skeleton to which the muscles are attached, and a soft, yielding, outer body-covering or skin; the crayfish has no internal skeleton, but has its body covered by a horny, firm body-wall to which the muscles are attached. The toad has its main nervous chain lying just beneath the dorsal wall of the body; the crayfish has its main nervous chain lying just above the ventral wall of the body. The toad has lungs and takes up oxygen from the air of the atmosphere; the crayfish has gills and takes up oxygen from the air which is mixed with the water. The toad has a single pair of jaws; the crayfish has several pairs of mouth-parts. The toad has four legs fitted for leaping; the crayfish has numerous legs fitted for crawling or swimming. The crayfish's body is composed of a series of body-rings or segments; the toad's body is a compact apparently unsegmented mass. The toad has eyes each with a single large lens and capable of moving in the head and of changing their shape and hence their focus; the crayfish's eyes are immovable and have a fixed focus, and are composed of hundreds of tiny eyes each with lens and special retina of its own. And so a long list of differences might be gone through with.
Resemblances between toad and crayfish.—But on the other hand there are many resemblances—resemblances both in structure and life-processes or physiology. Both toad and crayfish have organs for the prehension of food, its digestion and its assimilation. And these organs, the organs of the digestive system, while differing in details are alike in being composed principally of a long tube, the alimentary canal, running through the body, open anteriorly for the taking in of food, and open posteriorly for the discharge of indigestible useless matter. Both alimentary canals are divided into various special regions for the performance of the various special processes connected with the digestion and assimilation of food. Each is adapted for the special kind of food which it is the habit of the particular animal to take. The two sets of organs are essentially alike and have the same essential function or life-process to perform. But this process differs in the details of its performance, and the organs which perform this function and which constitute the digestive system of each are modified to suit the special habits or kind of life of the animal.
Both toad and crayfish have a heart with blood-vessels leading from it. In the case of the toad the heart is more complex than in the crayfish, and the system of blood-vessels is far more extensive and elaborate. But the heart and blood-vessels in both animals subserve the same purpose; their function is the circulation of the blood, this being the means by which oxygen and food are carried to all growing or working parts of the body, and by which carbonic acid gas and other poisonous waste products are brought away from these parts. But this function differs somewhat in its performance in the two animals, and the organs which perform the function are correspondingly modified in structural condition.