RESPIRATORY AND EXCRETORY SYSTEMS. 381 



branchial heart is contractile, and drives the venous blood 

 through the gills, whence purified it returns by two contractile 

 auricles into the ventricle. There are valves preventing 

 back flow from the ventricle to the auricles, or from the 

 arteries to the ventricle. Beside each branchial heart lies 

 an enigmatical glandular structure known as a "pericardial 

 gland," possibly an excretory or incipiently excretory organ. 

 The course of the blood differs from that in the mussel and 

 snail in this, that none returns to the heart except from the 

 respiratory organs. In the nephridial outgrowths around 

 the branchial veins the interesting parasite Dicyema is found. 



The blood is 

 feather-like gills 



Respiratory System. 



purified by being exposed on the two 

 which are attached within the water- 

 washed mantle cavity. 

 The water penetrates 

 them very thoroughly ; 

 the course of the blood 

 is intricate. At the base 

 of the gills there is some 

 glandular tissue, which 

 those impatient with enig- 

 mas have credited with 

 blood making powers. 



Fig. 122. — Diagram of circulatory 

 and excretory systems in a Decapod- 

 lilie Sepia. (After Pelseneer.) 



I, Gill ; 2, renal sac ; 3, afTerent branchial 

 vessel ; 4, branchial heart ; 5, abdominal 



vein ; 6, heart ; 7, pericardium ; 



Excretory System. 



The excretory system is diffi- 

 cult to dissect and to explain. 

 On each side of the anus there 

 is a little papilla through which 

 uric acid and other waste pro- 

 ducts ooze out into the mantle 

 cavity, and so into the water. 

 A bristle inserted into either of 

 these two papillre leatls into a 

 large sac — the nephritlial sac. 

 But the two sacs are united by 



two bridges, and they give off an unpaired dorsal elongation, which 



extends as far back as the reproductive organs. 



The dorsal wall of each nephridial sac becomes intimately associated 



with the branchial veins, and follows their outlines faithfully. It is 



likely that waste material passes from the blood through the spongy 



appendages into the nephridial sacs. 



S, genital 

 organ; 9, posterior aorta ; to, ''auricle"; 



11, glandular appendage of brarichial heart ; 



12, renal appendages of branchial vein ; 13, 

 external aperture of kidney ; 14, vena cava ; 

 15, anterior aorta; 16, bifurcation of vena 

 cava ; 17, reno-pericardial aperture. 



