XI 



L'HYLUM ARTHROPODA 



541 



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 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 car- j, 



bonic acid for oxygen ; from the gills by 

 efferent branchial veins to the branchiocar- 

 diac veins, thence into the pericardial sinus, 

 and so to the heart once more. 



It will be seen that the circulatory system 

 of the Crayfish consists 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 



with those of a Chsetopod, it would seem 



that the ophthalmic artery, heart, and dorsal 



abdominal artery together answer to a dorsal 



vessel, part of which has become enlarged 



and muscular and discharges the whole 



function of propelling the blood. The hori- 

 zontal portion of the sternal artery, together 



with the ventral abdominal, represent a 



ventral vessel ; while the vertical portion of 



the sternal artery is a commissure, developed 



sometimes on the right, sometimes on the 



left side, its fellow being suppressed. 



The blood when first drawn is colourless, 



but after exposure to the air takes on a 



bluish-grey tint. This is owing to the 



presence of a colouring matter called hcemo- 



cijanin, which becomes blue when combined with oxygen ; it is a 



respiratory pigment, and serves, like haemoglobin, as a carrier of 



oxygen from the external medium to the tissues. The hsemo- 



cyanin is contained in the plasma of the blood : the corpuscles are 



all colourless leucocytes. 



FIG. 448. Nervous system oi 

 Astacus fluviatilis. 



bg. sub-cesophageal gang- 

 lion ; eg. conmiissural 

 ganglion ; g. brain ; s, vis- 

 ceral nerve : sc, cesopha- 

 geal connective ; y, jiost- 

 uesophageal commissure ; 

 IV VIII, thoracic gang- 

 lia; 1-6, abdominal gang- 

 lia. (From Lang's Com- 

 parative Anatomy, after 

 Vogt and Yung.) 



