1896] BlICROSCOPICAL JOURNAL. 419 



general plan ( f the circulation is given here as a guide; 

 the student should locate as many of the vessels as pos- 

 sible from it. Ill the arterial system the blood is sent 

 from the bulbus arteriosus directly forward into a series 

 of pairs of aortic arches which follow the gill bones from 

 below upwards. In the roof of the mouth the aortic 

 arches unite, giving off carotid arteries to supply the 

 liead and then bending backwards to form the dorsal 

 aorta which runs backward the length of the body di- 

 rectly below the vertebral centra. At its anterior end 

 the aoi'ta gives rise to arteries coeliac axis and mesenteric 

 to the alimentary viscera, spleen and gonads, and small 

 arteries are given oft' throughout its length to the muscles 

 of the trunk and post-abdomen and to the kidneys renal 

 arteries. The venous blood from the head and all the mu8- 

 cular system and kidney is returned to the heart by 

 means of four veins in pairs, viz: the anterior and pos- 

 terior right and left cardinal veins', these empty into the 

 single auricle. The blood from the alimentary tube, 

 spleen and gonads does not go into this cardinal circula- 

 tion but is collected by means of the portal vein which 

 carries it to the liver whence it is taken to the heart by 

 the hepatic veins, two in number, opening into the auricle 

 independently of the cardinal veins.] 



11. The Uro-Genital System. — In addition to the 

 alimentary organs the body-cavity contains usually a 

 pair of large organs,, that are a i)art of the genital system. 

 They are not the true germinal tissue, from which the 

 eggs or sperm arise {gonads) but the greatly enlarged 

 ducts leading from the gonads to the exterior, tilled with 

 the jtroducts that have been thrown off from the gonads. 

 In the male they are very fine-grained and white, sper7n2- 

 ducts; in the female they are coarse grained and yellow, 

 oviducts. Examine an oviduct carefully, tracing it poster- 

 iorly till it passes to the exterior at the cloaca in common 



