neural tube pharynx, 



nofochord 

 snout 



junction of pharyngeal and atrial linings 

 subchordal coelom 



esophagus *"'" '■°y 



atrial pouch (longer on right side) 



caudal fin 



anus (on left side) 



velum with languets 

 cirri on oral hood 



hepatic diverticulum 

 ^°"°'^ LtapleuraHold Ventral body wall 



Figure 1-5. Gross internal relationships in Amphioxus. 



distance behind the esophagus a diverticulum (blindly end- 

 ing pouch) of the gut extends forward on the right side for 

 about one-third of the length of the pharynx. This hepatic 

 ("liver-like") diverticulum, carrying with it a pouch of the 

 coelom, projects into the atrium. Behind the diverticulum 

 the gut tapers toward the intestine, which leads to the anus 

 near the posterior end of the animal. The anus opens to the 

 left of the caudal fin, which is displaced to the right. 



The interior of the pharynx and the intestine is ciliated. 

 The pharyngeal cilia drive a mucous net from the ventral 

 endostyle or hypobranchial groove upward to the epibran- 

 chial groove. Here the net is rolled up and driven back into 

 the esophagus. In the gut the food materials are moved 

 about by the cilia during the process of digestion. 



The excretory system consists of a series of segmental 

 vesicles; into each of them opens a large number of soleno- 

 cytes, excretory cells containing a flagellum (Figure 1-4 D,E). 

 The vesicles open into the pharynx above the dorsal margin 

 of the shts. In addition to these segmental "nephridia," 

 there is a single tube extending forward to the right of the 

 notochord from just above the mouth; this is the nephridium 

 of Hatschek. 



In the ventrolateral angles of the pharyngeal region, there 

 are about 26 pairs of gonads (reproductive organs) in the 

 adult animal (Figure 1-4 F). These extend back to the 

 atrial pore. The sex cells are shed into the atrium and carried 

 to the exterior through the atrial pore. 



Each of the muscle segments lies in a connective tissue 



sheath. Near the notochord there is a small sclerocoel, a 

 term applied to the fluid-filled cavities along the axis of the 

 animal. The sclerocoels also enclose the rays of the dorsal 

 and ventral (posterior to atriopore) fin folds. 



The notochord is well developed and has a strong outer 

 sheath and inner elastic membrane (Figure 1-4 A). Just 

 inside the membrane are the nuclei of the chordal cells. 

 The central cells are discs with a large central vacuole; 

 within the notochord they are stacked from front to rear. 

 There are ventral and dorsal canals just within the sheath 

 which contain small, free cells, the chordal corpuscles. 



Amphioxus has a well-developed circulatory system (Fig- 

 ure 1-6). There is a ventral blood vessel which collects from 

 the digestive tract and extends forward to the hepatic diver- 

 ticulum and along the underside of that structure. Here it 

 breaks down into smaller vessels which permeate the wall of 

 this diverticulum and are recollected by a dorsal vessel. This 

 dorsal vessel hooks forward beneath the pharynx and here 

 receives the so-called posterior and anterior cardinal vessels 

 draining the ventrolateral body wall. The main channel 

 leads forward below the endostyle sending branches into the 



gill bars. 



The blood is collected dorsally by paired aortae which 

 join behind the pharynx to form a midline dorsal aorta. 

 This has branches extending through the myosepta both 

 dorsally and ventrally to the body wall (Figure 1-4 A). The 

 myosepta are sheets of connective tissue separating the blocks 

 of muscle, the myomeres, one from another. The dorsal 



serves Hatschek's ne| 



tongue bar vessels left and right radices aortae 



exterior septal vessel 

 ' internal septal vessel H dorsal oorto 



dorsal segmental artery opens into myocoel 

 ventral segmental artery 

 ntestinal artery 



intestinal reticulum 



coelomic vessel 



common cardinal 

 sinus venosus 

 hepatic vein (c) 



anterior cardinal 'hepatic reticulum 



Figure 1 -6. Diagrammatic circulatory syster,, of Amphioxus. Controct.le vessels are indicated (c 



bulbule (c 

 endostylar vessel (c) 



gonad vessel 



postcardinal channel 



right postcardinal continues 

 as caudal vein) 



'subintestinol channel (c) 

 parietal reticulum 



8 • CHORDATES, PROTOCHORDATES, AND THEIR RELATIONSHIPS 



