PHYLUM CHORDATA 



399 



oxygenated during its passage through the branchial arteries. 

 The direction of the blood flow, backward in the dorsal and for- 

 ward in the ventral vessel, is like that of the vertebrates (p. 407), 

 but just the reverse of that in annelids and arthropods (see pp. 

 221 and 283). 



THE CCELOM. The coelom arises from five embryonic pouches 

 of the primitive digestive tract *as in Balanoglossus (p. 388), but 

 is difficult to make out in the adult. The position of the ccelomic 

 cavities is shown in Fig. 343, coel, and Fig. 342, d.co. 



EXCRETORY SYSTEM. The excretory organs are ciliated 

 nephridia (Figs. 342-343, nph) situated near the dorsal region 

 of the pharynx. The nephridia connect the dorsal ccelom (Fig. 



342, d.co) with the atrial cavity. A pair of brown funnels (Fig. 



343, br.f), one on either side and dorsal to the intestine in the 

 region of myotome twenty-seven, may also be excretory organs. 



NERVOUS SYSTEM. Amphioxus possesses a central nerve-cord 

 (Fig. 343, sp.cd ; Fig. 342, nt) lying entirely above the alimen- 

 tary canal (compare annelids, p. 216, and arthropods, p. 285). 

 It rests on the notochord and is almost as long. A minute canal 

 (Fig. 343 cent.c) traverses its entire length and enlarges at the 

 anterior end into a cerebral vesicle (en.coe) which is the only trace 

 of a brain present. An olfactory pit (olf.p) opens into this 

 vesicle in young specimens. At the anterior end of the nerve- 

 cord is a mass of pigmented cells forming an eye-spot (e.sp). 

 Two pairs of sensory nerves arise from the cerebral vesicle, and 

 supply the anterior region of the body. The rest of the nerve- 

 cord gives off nerves on opposite sides, but alternating with one 

 another. These nerves are of two kinds: (i) dorsal nerves with 

 a sensory function which pass to the skin, and (2) ventral nerves 

 with a motor function which enter the myotomes. The sense- 

 organs include the olfactory pit, eye-spot, and sensory cells in 

 the ectoderm, on the cirri, and on the velar tentacles. 



REPRODUCTION. In Amphioxus the sexes are separate. 

 The twenty-six pairs of gonads (Fig. 341, "g l , g 26 ; Fig. 342, go) 

 project into the atrium. The germ-cells are discharged into the 



