440 



CHORDATA 



to a rudimentary cerebral vesicle, with a pigment spot, the rudimentary 

 eye (an), but other places in the spinal cord are sensitive to light. The 

 neural canal long remains open in front, the opening (neuropore) being at 

 the bottom of a pit, once regarded as an olfactory organ. 



The alimentary canal begins with the oval mouth (0) , surrounded by 

 cirri; then comes a fold (velum], followed by the pharynx, perforated by 

 numerous gill clefts, which extend over a third of the alimentary canal. 

 Between the clefts are elastic gill arches (fig. 494, kb) to support the walls. 

 In the young the clefts open directly to the exterior; then a fold grows 



-sn 



S22 



-I 



FIG. 494. Section of the gill region of Amphioxus (after Lankester and Boveri). 

 a, aorta descendens; b, peribranchial space; c, notochord; co, ccelom (branchial body 

 cavity); e, hypobranchial groove, beneath it the aorta ascendens; g, gonad; kb, gill 

 arches; kd, pharynx; /, liver; m, muscles; n, nephridia, on the left with an arrow; r, 

 spinal cord; sn, spinal nerve; sp, gill slit. 



down on either side, the two folds uniting below to enclose a peribranchial 

 chamber (b) from which the water escapes by an opening, atriopore (493,), 

 behind the middle of the body. On the floor of the pharynx is the hypo- 

 branchial groove, a ciliated tract which conducts food to the digestive part 

 of the canal. This ends at the anus (a) , on the left side of the body and has 

 a liver connected with it in front, extending into the peribranchial chamber 

 (figs. 493, 494, b). The vascular system, with colorless blood, consists of 

 a dorsal arterial (a) and a ventral venous trunk connected by lateral loops 



