- 140 Invertebrate Zoology. 



the peripharyngeal bands to the dorsal lamina, and thence 

 posteriorly along the " roof ' of the pharyngeal sac to the 

 opening into the oesophagus. In certain species the oesoph- 

 agus opens into an enlarged stomach, and frequently 

 accessory digestive glands are present. A median fold, the 

 typhlosole, partially divides the lumen of the intestine. 



The Excretory System. - Besides the renal organ 

 already noted, there are certain small anomalous collec- 

 tions of vesicles around the intestine and in the tunic that 

 are supposed to perform excretory functions. The neural 

 gland is also supposed by some to function as a renal organ. 



The Circulatory System. In the Tunicata there are 

 no true arteries or veins. The heart is a sac-like organ 

 which forces the blood intermittently towards the posterior 

 dorsal portion of the animal, and then in the opposite direc- 

 tion towards the anterior ventral. It is surrounded by a 

 fold, the pericardium. During the respiratory circulation, a 

 ventral trunk conveys the blood from the heart along the 

 ventral portion of the animal, below the endostyle, and gives 

 off lateral branches to the transverse vessels of the branchial 

 sac. From these vessels the blood passes into the fine, 

 irregularly disposed interstigmatic vessels, the meshes of 

 which surround the stigmata. The longitudinal vessels in 

 Molgula are arranged on the branchial folds already noted. 



When the current is from the ventral towards the dorsal 

 portion of the animal, the blood passes through certain 

 vessels to the mantle, the digestive and reproductive organs. 

 The blood from these organs is then collected in a dorsal 

 trunk, which lies along the mid-dorsal line of the branchial sac, 

 and from thence it passes into the transverse vessels. There 

 are then, during this cycle, three systems, ist, Branchio- 



