CRUSTACEA 



97 



bathes the tissues, then goes into the larger median ventral 

 sinus below the thorax and abdomen, from which it is conducted 

 to the gills. After being conveyed to the gill filaments, where 

 it is aerated, it is returned to the heart through the pericardial 

 sinus. The blood enters the heart, or dorsal vessel, through 

 three pairs of openings, one on each side, a pair on the top, and 

 another pair below. Valves prevent the blood from returning 

 through these openings. 



C la- 



sh 



Fig. 71. Astacus fluvialilis. The heart (x 4). A, From above; B, 

 from below; C, from the left side: a.a., Antennary artery; a.c., alae cordis, 

 or fibrous bands connecting the heart with the walls of the pericardial 

 sinus; b, bulbous dilatation at the origin of the sternal artery ; h.a., hepatic 

 artery; La., lateral valvular apertures; o.a., ophthalmic artery; s.a., superior 

 valvular apertures; s.a.a. superior abdominal artery; st.a., sternal artery, 

 in B cut off close to its origin. (After Huxley.) 



Respiration. The plume-like gills are attached to /the 

 basal joints of the legs. They are situated in partially 

 closed chambers between the body wall and the carapace. 

 The water is drawn in and out by the " gill-bailers," parts of 

 the second maxillae, in their vibration back and forth. In 

 passing over the gills the water is separated from the blood by 

 an extremely thin membrane. Through this membrane the 

 carbon dioxid is thrown off and oxygen taken into the blood. 



Nervous System. Several ganglia unite to form the supra- 



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