202 AN INTRODUCTION TO ZOOLOGY 



suspended a number of ameboid cells, the blood corpuscles or 

 amebocytes. The principal functions of the blood are the 

 transportation of food materials from one part of the body to 

 another, of oxygen from the gills to the various tissues, of carbon 

 dioxide to the gills, and of urea to the excretory organs. If a 

 crayfish is wounded, the blood, on coming in contact with the air, 

 thickens, forming a clot. It is said to coagulate. This clogs the 

 opening and prevents loss of blood. The chelipeds and other 

 walking legs of the crayfish have a breaking point near their 

 bases. When one is injured the animal may break the limb at 

 this point and lessen the blood flow, since only a small space is 

 present in the appendage at this particular spot, and coagulation, 

 therefore, takes place very quickly. 



BLOOD VESSELS. The principal blood vessels are a heart, 

 seven main arteries, and a number of spaces called sinuses. 



HEART. The heart (Fig. 101, 2p) is a muscular-walled, saddle- 

 shaped sac lying in the pericardial sinus in the median dorsal part 

 of the thorax. It may be considered as a dilatation of a dorsal 

 vessel resembling that of the earthworm. Six elastic ligaments, 

 two anterior, two posterior, and two running along the ventral 

 border of each lateral surface, fasten it to the walls of the peri- 

 cardial sinus. Three pairs of valvular apertures, called ostia, 

 one dorsal and two lateral, allow the blood to enter from the 

 surrounding sinus (174). 



ARTERIES. Five arteries arise from the anterior end of the 

 heart (Fig. 101). 



(i) The ophthalmic artery (34) is a median dorsal tube passing 

 forward over the stomach, and supplying the cardiac portion, 

 the oesophagus, and head. 



(2, 3) The two antennary arteries (35) arise one on each side 

 of the ophthalmic artery, pass forwards, outwards, and down- 

 wards, and branch, sending a gastric artery to the cardiac part 

 of the stomach, arteries to the antennse, to the excretory organs, 

 and to the muscles and other cephalic tissues. 



(4, 5) The two hepatic arteries (36) leave the heart below 



