490 TEXTBOOK OF ZOOLOGY 



leads into the anterior section of the small intestine or duodenum, 

 which includes the first S-shaped turn of the tube. The more coiled 

 part of the small intestine following this is the ileum. This empties 

 into the short but somewhat broadened large intestine, which opens 

 into the cloaca, the common receptor of faecal matter from the intes- 

 tine and urinogenital products from the wolffian ducts, urinary blad- 

 der and miillerian ducts. The urinary Madder is a thin-walled sac 

 hanging at the ventral side of the cloaca, whose lumen it joins. 



The liver is an elongated, dark-colored and somewhat serrated or- 

 gan lying in the ventral portion of the body cavity. The gall bladder 

 is a membranous sac attached to the margin of the liver (usually at 

 the right side). The bladder is connected with the duodenum by a 

 hile duct which is obscured by a mass of pancreatic tissue. The pan- 

 creas is divided into slender lobes and lies in the vicinity of the junc- 

 tion of stomach and duodenum. One lobe extends to the tip of the 

 spleen which lies dorsolateral to the stomach. Another slender lobe 

 extends posteriorly in the mesentery that supports the anterior part 

 of ileum and the mesenteric vein. Both of these lobes join the mass 

 of pancreatic tissue around the bile duct and the pancreatic ducts 

 enter the duodenum at this level. 



Circulatory System 



There is a partial conversion from the straight branchial type of 

 circulation of the fish to the pulmonary type of the terrestrial verte- 

 brates, in that the number of functional aortic arches is reduced, 

 pulmonary vessels are added to supply the lungs, and the atrium of 

 the heart is divided into two parts to keep the systematic and aerated, 

 pulmonary blood partially separated. With certain modification of 

 the situation in fish, the system consists of heart, aortic arches, sys- 

 temic and pulmonary arteries, as well as systemic, renal portal, hepatic 

 portal, and pulmonary veins. The heart consists of right and left 

 atria (auricles) and one ventricle with the two usual accessory 

 chambers, the sinus venosus which joins the right atrium and the 

 conus arteriosus which leads from the ventricle to the ventral aorta. 

 The right atrium receives the systemic blood and the left atrium, the 

 pulmonary blood by way of pulmonary veins. Blood in passing 

 through the heart may be traced by entering the sinus venosus from 

 systemic veins, pass by way of sinu-atrial (sinuauricular) valve to 

 right atrium, thence through the atrioventricular valve to the ventricle 

 which it enters simultaneously with aerated blood from left atrium. 



