XI 



PHYLUM AKTHROPODA 



539 



3 ;.,. 



of a glandular network formed of anastomosing canals, and com- 

 municating in its turn with (3) a white portion (w. p.), formed of a 

 single tube partly converted into a sponge-work by ingrowths 

 of its walls. The whole organ is lined by glandular epithelium, 

 and the white portion discharges into a thin- walled sac or urinary 

 bladder (bl.) which opens by a duct (d.) on the proximal segment of 

 the antenna. The glands already referred to as occurring in the 

 gills are also supposed to have an excretory function. 



The circulatory organs are in a high state of development. 

 The heart (Figs. 443, 446, h.) is situated in the dorsal region of the 

 thorax, and is a roughly 

 polygonal muscular organ 

 pierced by three pairs of 

 apertures or ostia (o.), guarded 

 by valves which open inwards. 

 It is enclosed in a spacious 

 pericardial sinus (Fig. 446, 

 pc.), which contains blood. 

 From the heart spring a f 

 number of narrow tubes, 

 called arteries, which serve to 

 convey the blood to various 

 parts of the body. At the 

 origin of each artery from the 

 heart are valves which allow 

 of the flow of blood in one 

 direction only, viz., from the 

 heart to the artery. From 

 the anterior end of the heart 

 arise five vessels the median 

 ophthalmic artery (Fig. 443, 



Oa.), which passes forwards to FI,;. 44C. Transverse section of thorax of Cray- 



tVip PIT-PC -nairprl fimiovtvinfti fish, diagrammatic, abm. ventral abdominal 



eyes , pal amennary muscles ; &/. leg ; bm. ventral nerve cord : d. 



arteries (aa ) fifoinp: to the intestine ; dbm. dorsal muscles of abdomen ; 



-. v ep. wall of thorax ; h. heart ; k. gills ; kd. gill- 



antennuies, antenna?, green cover ; I. digestive gland ; ov. ovary ; pc. 



crl.mrlc Arn ar\f] -pnrlinrr r># pericardial sinus ; sa.sn., sternal artery ; vs. 



gianClS, <KC., ana Sending On ventral sinus. The arrows show the direction of 



branches to the olzzard ' * ne blood-current. (From Lang's Comparative 



and paired hepatic arteries, 



going to the digestive glands. The posterior end of the heart 

 gives off two unpaired arteries practically united at their 

 origin, the dorsal abdominal artery (oaa.). which passes back- 

 wards above the intestine, sending branches to it and to the 

 dorsal muscles ; and the large sternal artery (sa.), which extends 

 directly downwards, indifferently to right or left of the intestine, 

 passing between the connectives uniting the third and fourth 

 thoracic ganglia, and then turns forwards and runs in the sternal 

 canal, immediately beneath the nerve-cord, sending off branches 



