THE ANIMAL KINGDOM 



443 



of the branchial chamber. Above the dorsal ends of the gill 

 slits are numerous paired nephridial tubes opening into the 

 branchial chamber. In the mid-dorsal line and also in the mid- 

 ventral line of the pharynx is a ciliated groove, thought to be 

 homologous with the endostyle of tunicates. Back of the 

 pharynx the intestine continues as a straight tube to an anal 

 opening near the posterior end of the animal, but to the left 

 of the mid-line. The liver is a diverticulum from the anterior 

 end of the intestine, extending forward to the right of the 

 pharynx. Its walls are said to produce a digestive fluid. 

 The digestive tract lies in a body cavity lined by coelomic 



nt 



7& 



n 



t an 



i s I J a P 



Fig. 265. — Diagram of young Amphioxus shortly after metamorphosis. 

 a, atrium; an, anus, opening asymmetrically on the left side; ap, atropore; c, 

 oral cirri, surrounding mouth; e, eyespot; gs, gill septa; i, intestine; I, liver; n, 

 notochord; nt, neural tube; o, olfactory pit. (Based on Leuckart-Nitsche wall 

 chart.) 



epithelium. The musculature of the trunk is conspicuously 

 segmented into V-shaped somites as in fishes. 



The ventral surface of the body is flattened in the region of 

 the branchial chamber, with slightly projecting edges on each 

 side, known as metapleural folds. A dorsal fin runs the length 

 of the body and connects posteriorly with a caudal fin around 

 the tail. Ventrally the caudal fin is joined with the ventral 

 fin, which extends forward to the atriopore. 



There is no heart, auditory organ, skull, or vertebral column. 

 The notochord is a well-developed rod, tapered at the ends, 

 lying between the neural tube above and the digestive tract 

 below, and extending the entire length of the body. The 

 anterior end of the neural tube is slightly enlarged into a region 

 corresponding to the brain of vertebrates. The remainder of 

 the neural tube is the spinal cord. A median olfactory 

 pit, located at the anterior end of the neural tube, opens 

 externally on the dorsal surface just back of the tip of the snout. 

 Nerves extend from the neural tube to all parts of the body. 



