xxvii CIRCULATION 339 



stem. The eighteen efferent branchial veins open into six 

 branchio-cardiac veins (br. c. v), which pass dorsally in close 

 contact with the lateral wall of the thorax and open into 

 the pericardial sinus. 



The whole of this system of cavities is full of blood, and 

 the heart is rhythmically contractile. When it contracts the 

 blood contained in it is prevented from entering the peri- 

 cardial sinus by the closure of the valves of the ostia, and 

 therefore takes the only other course open to it, viz., into 

 the arteries. When the heart relaxes, the blood in the 

 arteries is prevented from regurgitating by the valves at 

 their origins, and the pressure of blood in the pericardial 

 sinus forces open the valves of the -ostia and so fills the 

 heart. Thus in virtue of the successive contractions of the 

 heart, and of the disposition of the valves, the blood is kept 

 constantly moving in one direction, viz., from the heart by 

 the arteries to the various organs of the body, where it 

 receives carbonic acid and other waste matters ; thence by 

 sinuses into the great sternal sinus ; from the sternal sinus 

 by afferent branchial veins to the gills, where it exchanges 

 carbonic acid for oxygen ; from the gills by efferent branchial 

 veins to the branchio-cardiac veins, thence into the peri- 

 cardial sinus, and so to the heart once more. 



It will be seen that the circulatory system of the crayfish 

 consists of three sections (i) the heart or organ of pro- 

 pulsion ; (2) a system of out-going channels, the arteries, 

 which carry the blood from the heart to the body generally ; 

 and (3) a system of returning channels some of them, the 

 sinuses, mere irregular cavities, others, the veins, with 

 definite walls these return the blood from the various 

 organs back to the heart. The respiratory organs, it should 

 be observed, are interposed in the returning current, so that 

 blood is taken both to and from the gills by veins. 



Z 2 



