PHYLUM ARTHROPODA 



207 



and supplying the cardiac portion, the eso- 

 phagus, and the head. 



2, 3. The two antennary arteries arise one 

 on each side of the ophthalmic artery. They 

 pass forward, outward, and downward, and 

 then branch, sending a gastric artery to the 

 cardiac part of the stomach, and arteries to 

 the antennae, excretory organs, muscles, and 

 to other cephalic tissues. 



4, 5. The two hepatic arteries leave the 

 heart below the antennary arteries. They 

 lead directly to the digestive glands. 



6. The dorsal abdominal artery is a me- 

 dian tube leading backward from the ventral 

 part of the heart and supplying the dorsal 

 region of the abdomen. It branches near its 

 point of origin, giving rise to the sternal 

 artery; this leads directly downward, and, 

 passing between the nerve cords connecting 

 the fourth and fifth pairs of thoracic ganglia, 

 it divides into two arteries (Fig. 114). One 

 of these, the ventral thoracic artery, runs 

 for^vard beneath the ner\'e chain and sends 

 branches to the ventral thoracic region and 

 to appendages 2 to 12; the other, the ventral 

 abdominal artery, runs backward beneath 

 the nerve chain and sends branches to the 

 ventral abdominal region. 



Sinuses. The blood passes from the small- 

 est arteries into spaces lying in the midst 

 of the tissues, called sinuses. The pericardial 

 sinus has already been mentioned. The 

 thorax contains a large ventral blood space, 

 the sternal sinus, and a number of branchio- 

 cardiac sinuses that lead from the bases of 

 the gills, up the inner sides of the thoracic 

 wall, to the pericardial sinus. A perivisceral 

 sinus surrounds the digestive tract in the 

 cephalothorax. 



Blood flow. The heart by means of rhyth- 

 mic contractions forces the blood through 

 the arteries to all parts of the body. Valves 

 are present in every artery where it leaves 

 the heart; they prevent the blood from 

 flowing back. The finest branches of these 

 arteries, open into spaces between the tis- 

 sues, and the blood eventually reaches the 

 sternal sinus. From here it passes into 



afferent channels of the gills and into the 

 gill filaments, where the carbon dioxide is 

 given off and oxygen is taken in from the 

 water in the branchial chambers. It then 

 returns by way of the efferent gill channels, 

 passes into the branchiocardiac sinuses, 

 thence to the pericardial sinus, and finally 

 through the ostia into the heart. The valves 

 of the ostia allow the blood to enter the 

 heart, but prevent it from flowing back into 

 the pericardial sinus. 



The crayfish thus has an open (lacunar) 

 blood system in which the blood is distrib 

 utcd to blood spaces (sinuses) before being 

 returned to the heart. There are no veins as 

 in vertebrates. 



Respiratory system 



Breathing in the crayfish is by means of 

 plumy gills. Between the branchiostegites 

 and the body wall are the branchial cham- 

 bers containing the gills (Fig. 116). At the 

 anterior end of the branchial chamber there 

 is a channel in which the gill bailer ( scaph- 

 ognathite) of the second maxilla moves 

 back and forth, forcing the water out 

 through the anterior opening. Water flows 

 in through the posterior opening of the 

 branchial chamber and ventrally. 



There are two rows of gills, named accord- 

 ing to their points of attachment. The outer- 

 most, the podobranchs, are fastened to the 

 coxopodites of certain appendages; and the 

 inner double row, the arthrobranchs, arise 

 from the membranes at the bases of these 

 appendages. In Astacus there is a third row, 

 the plcurobranchs, attached to the walls of 

 the thorax. The podobranchs consist of a 

 basal plate covered with delicate setae and 

 a central axis bearing a thin, longitudinally 

 folded, corrugated plate on its distal end, and 

 a featherlike group of branchial filaments. 

 Each arthrobranch has a central stem, on 

 each side of which extends a number of fila- 

 ments, causing the entire structure to re- 

 semble a plume. Attached to the base of 

 the first maxilliped is a broad thin plate, the 

 epipodite (Fig. 115), which has lost its 



