554 ZOOLOGY sect. 



contained in it is prevented from entering the pericardial 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 con- 

 stantly moving in one direction — viz., from the heart by the 

 arteries to the various organs of the body, where it receives car- 

 bonic 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 branchiocardiae 



Fio. 439.— Diagram of the circulation in the Crayfish ; heart and arteries scarlet, veins and 

 sinuses containing non-aerated blood, blue ; those containing aiji-ated blood, pink. n. artery ; 

 (ff.br. I. afferent branchial vein; br.f:.v. branchio-cardiac vein; ef.br.v. efferent branchial 

 vein; ht. heart ;. pccLn. pericardial sinus;*, sinus; st.s. sternal sinus; ijl. ostium with 

 valves ; v^. arterial valves. The arrows show the direction of the current. 



veins, thence into the pericardial sinus, and so to the heart once 

 more. 



It will be seen that the circulatory system of the Crayfish con- 

 sists of three sections — (1) the heart or organ of propulsion; (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 return- 

 ing channels, some of them, the sinuses, mere irregular cavities ; 

 others, the veins, with definite walls, which return it 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. 



Comparing the blood-vessels of Astacus Avith those of a 

 Chsetopod, it would seem that the ophthalmic arterj^, heart, and 

 dorsal abdominal artery together answer to a dorsal vessel, part 

 of which has become enlarged and muscular and discharges the 



