Classification of Chordates: Protochordata 403 



epipharyngeal groove. Particles of food, swept into the pharynx by the 

 respiratory current of water, are caught on the moving bands of mucus 

 and thus transported back into the intestine. 



No stomach is differentiated. The pharynx opens into a straight 

 tube, the intestine, at whose anterior end is a ventral tubular glan- 

 dular diverticulum (hepatic cecum), possibly corresponding to the 

 liver of vertebrates. 



The relations of coelom and peritoneum are essentially as in verte- 

 brates. The sexes are separate. The gonads, about 25 segmental! y 

 arranged pairs of them, are embedded in the outer wall of the atrial 

 cavity (Fig. 314). There are no genital ducts. When the eggs or sperms 

 are mature, temporary rupture of the enclosing wall of each gonadic 

 pouch permits escape of the genital products into the atrial cavity, 

 whence they find exit via the atriopore. The apparently extraordinary 

 position of the gonads results from the manner of origin of the atrial 

 cavity. A pair of lateral longitudinal folds (metapleural folds) of the 

 embryonic body- wall grow outward and downward (Fig. 314). Later 

 the two folds become joined by a transverse horizontal partition (Fig. 

 316A). The external space thus enclosed between the folds and the 

 body-wall becomes the atrial (peribranchial) cavity, which is therefore 

 lined by ectoderm. Incomplete union of the folds at their hind ends 

 leaves the atriopore. The gonads, originally situated on the internal 

 surface of the body-wall, are carried down into the atrial wall by the 

 downgrowth of the atrial folds. 



Coelom Brown funnel 



Notochord j Myomere / Dorsal fin 



Spinal cord 

 Brain 



A 



/ Myom 



-i/M 



"*— - ' ~ 



Fig. 313. Arnphioxus; longitudinal view showing the major anatomic features. 

 The body-wall, spinal cord, and notochord are cut in the median plane. A short 

 anterior region of the pharynx is cut in the median plane, revealing the internal 

 openings of four gill-clefts of the right side. Posterior to this region may be seen five 

 gill-clefts opening into the atrium whose external wall has been removed along with 

 the body-wall. (In the fully adult animal the gill-clefts are much more numerous 

 than shown in the figure.) The intestine is not cut, and its posterior end bends 

 slightly to the left so that the anus is on the left side of the thin caudal fin. The 

 liver extends forward on the right side. The "brown funnel" is one of a pair of 

 tubules, presumably excretory, opening into the atrium. 



