106 MANUAL OF ANIMAL BIOLOGY 



dorsal body wall, and gives off branches which supply the muscle 

 tissue of this region. A large branch, the sternal artery, arises 

 from the abdominal artery just after it leaves the heart. The 

 sternal artery continues ventrally, at right angles to the abdominal 

 artery, till it reaches the ventral body wall. The ventral nerve cord 

 of the Crayfish at this point splits into two cords which are united 

 at the segmental ganglia. The sternal artery passes between the 

 two nerve cords and, underneath them, divides into (a) the ven- 

 tral thoracic artery, which runs anteriorly under the nerve 

 cord, and (6) the ventral abdominal artery, which runs pos- 

 teriorly and occupies a corresponding position. 



There are six openings (ostia x ) in the muscular walls of the heart 

 through which blood passes into the heart from the surrounding 

 pericardial sinus. One pair of these ostia opens through the dorsal 

 wall, one pair through the ventral wall, and one pair opens through 

 the sides. All of the ostia are provided with simple valves which 

 prevent the outflow of blood. 



Having considered the heart and the vessels leading from it, we 

 are now in a position to note the position of the large blood spaces, 

 or sinuses, which are present in various regions of the body and 

 in which the blood is collected and finally returned to the heart. 

 The pericardial sinus, which surrounds the heart, has already 

 been noted. The main sinus of the body is known as the sternal 

 sinus. It is situated on the ventral side of the body in the region 

 of the thorax. Leading from the sternal sinus are a number of 

 channels which run into the organs for aerating the blood, the 

 gills. After passing through the gills the blood flows dorsally 

 into the pericardial sinus. The alimentary canal in the cephalo- 

 thorax is surrounded by the perivisceral sinus. 



The general course of the circulation of the blood is as follows : 

 The blood, which passes from the surrounding pericardial sinus 

 through the paired ostia and into the heart, is forced out by the 

 rhythmical contractions of the muscular walls into the various 

 arteries mentioned above. Thus the heart drives the blood both 

 anteriorly (median, antennary, hepatic, and ventral thoracic ar- 

 teries) and posteriorly (dorsal and ventral abdominal arteries) to 

 supply all regions of the body. In the tissues the blood is gradu- 

 ally collected in the sinuses, all of which finally open into the large 

 sternal sinus. The blood which flows to the sternal sinus from 



1 Singular, ostium. 



