The Food Tube.—Is the stomach in the head portion of the cephalothorax or in the thoracic portion? (Figs. [88],89). Is the stomach large or small? What is its general shape? Does the gullet lead upward or backward? Is it long or short? (Fig. [88].) The mid tube, which is the next portion of the food tube, is smaller than the stomach. On each side of it are openings from the bile ducts which bring the secretion from the digestive gland, sometimes called the liver. Does this gland extend the whole length of the thorax? Is it near the floor or the top of the cavity? The third and last portion of the food tube is the intestine. It extends from the thorax to the vent. Is it large or small? Straight or curved? The powerful flexor muscles of the tail lie in the abdomen below the intestines. Compare the size of these muscles with the extensor muscle above the intestine (Fig. [90]). Why this difference? Does the food tube extend into the telson? Locate the vent (Fig. [90]).
Fig. 91.—Showing heart and main blood vessels.
The Circulation.—The blood is a liquid containing white corpuscles. It lacks red corpuscles and is colourless. The heart is in the upper part of the thorax. It is surrounded by a large, thin bag, and thus it is in a chamber (called the pericardial sinus). The blood from the pulmonary veins enters this sinus before it enters the heart. The origin of this pericardial sinus by the fusing of veins is shown in Fig. [130]. Does one artery, or do several arteries, leave the heart? There is a larger dorsal artery lying on the intestine and passing back to the telson; there are three arteries passing forward close to the dorsal surface (Figs. [89], [91]). One large artery (the sternal) passes directly downward (Figs. [88], [91]), and sends a branch forward and another backward near the ventral surface. The openings into the heart from the sinus have valvular lips which prevent a backward flow of blood into the sinus. Hence, when the heart contracts, the blood is sent out into the several arteries. The arteries take a supply of fresh blood to the eyes, stomach, muscles, liver, and the various organs. After it has given oxygen to the several organs and taken up carbon dioxide, it returns by veins to pass through the gills on each side, where it gives out the useless gas and takes up oxygen from the water. It is then led upward by veins into the pericardial sinus again.
Fig. 92.
The central nervous system consists of a double chain of ganglia (Fig. [92]). This main nerve chain lies along the ventral surface below the food tube (Fig. [90]), except one pair of ganglia which lie above the œsophagus or gullet (Fig. [88]), and are called the supra-œsophageal ganglia, or brain.
Crustacea.—The crayfish and its kindred are placed in the class called Crustacea.
Fig. 93.—Crab from below.