Phylum Chordata 563 



From the branchial sac, the digestive tract continues with a short 

 esophagus leading to a much dilated stomach. The stomach opens into a 

 long intestine which loops and ends in the anus near the excurrent siphon. 

 There is a digestive gland connected by a duct to the stomach. 



The excretory system of this animal is not well understood, but it 

 is thought to be a small, brownish sac attached to the inner wall of the 

 body. Since there is no duct from this sac, its function must be prin- 

 cipally the concentration of metabolic wastes. A circulatory system is 

 present in the form of a heart and associated blood vessels. The heart 

 is an elongate, translucent tube located near the excretory organ. A 

 blood vessel passes from it at each end. The heart is quite unique in its 

 functioning as it beats by means of peristaltic waves which change their 

 direction of movement. The waves pass in one direction for a period, 

 then stop and reverse. 



The nervous system is much reduced, being simply a small cerebral 

 ganglion which is located dorsally in the mantle between the two siphons. 

 From it, a few nerves pass to the remainder of the body. 



The tunicate is hermaphroditic with the gonads being fastened to 

 the inner wall of the mantle. The testes and ovaries are located close 

 to each other with a pair on each side. There are separate ducts for the 

 eggs and sperm. These ducts open into the atrial cavity near the anus 

 at the base of the excurrent siphon. Eggs are discharged into the water 

 and fertilization occurs outside the animal. 



Fig. 187. — The tadpolelike larva of the tunicate. 



The typical tadpolelike larva develops from the fertilized egg within 

 twenty-four hours. This peculiar larva has a long tail containing the 

 notochord, has a dorsal tubular nervous system, lateral gill slits, a cir- 

 culatory system, coelom, and sense organs including an eye and a primi- 

 tive ear. At the anterior end, there are mucus-secreting glands, the ad- 



