388 ^"^ ANIMAL KINGDOM 



Food is entrapped within the pharynx in mucus secreted by an 

 endostyle just as it was in urochordates. Water in the pharynx escapes 

 into an ectodermally lined atrium through nearly two hundred gill 

 slits. Gill bars, supported by delicate skeletal rods, lie between the 

 slits. At one stage in development the gill slits are U-shaped, and re- 

 semble those of hemichordates (Fig. 19.4), a detail that may point to an 

 affinity between these animals, but in Amphioxus the tonguelike process 

 that causes the slit to be U-shaped subsequently continues its down- 

 ward growth and completely subdivides the slit. Some gas exchange 

 occurs in the pharynx, but the skin is the main respiratory surface. 

 The pharynx, therefore, is primarily a food-gathering device. 



After leaving the pharynx, the food enters a short esophagus, a 

 midgut, and finally an intestine, which opens at the surface through an 

 anus. The intestine terminates before the end of the body, so there is a 

 postanal tail as in vertebrates. A prominent midgut caecum, which 

 produces digestive enzymes, extends from the floor of the midgut for- 

 ward along the right side of the pharynx. 



Absorbed food and other substances are distributed by a circulatory 

 system. A series of veins returns blood from the various parts of the 

 body to a sinus which is located ventral to the posterior part of the 

 pharynx, and may be comparable to the posterior part of the vertebrate 

 heart. A muscular heart, however, is not present, and the blood is 

 propelled by the contraction of the arteries. A ventral aorta extends from 

 the sinus forward beneath the pharynx, and leads into branchial ar- 

 teries that travel dorsally through the gill bars into a pair of dorsal 

 aortas. The dorsal aortas, in turn, carry the blood posteriorly to spaces 

 within the tissues. True capillaries are absent, but the general direction 

 of blood flow, i.e., anteriorly in the ventral part of the body and pos- 

 teriorly in the dorsal part, is similar to that of a vertebrate and different 

 from that of other lower chordates. 



The excretory organs are segmentally arranged, ciliated proto- 

 nephridia (p. 208) that lie dorsal to certain gill bars and open into the 

 atrium. 



The nervous system of Amphioxus consists of a tubular nerve cord 

 located dorsal to the notochord. Its anterior end is differentiated slightly, 

 but does not expand to form a brain. Paired, segmental nerves, con- 

 sisting of dorsal and ventral roots, extend into the tissues. The roots 

 remain separate and do not unite. The ventral roots go directly into 

 the myomeres, and the dorsal roots pass between myomeres to supply 

 the skin, gut wall and ventral parts of the body. Amphioxus is sensitive 

 to light, and to chemical and tactile stimuli, but elaborate sense organs 

 are not present. The cirri on the oral hood and a flagellated pit in 

 the skin near the front of the nerve cord appear to be chemoreceptors. 

 Photoreceptive cells, which are partly masked with pigment, lie in the 

 nerve cord. The prominent pigment spot at the anterior end of the 

 cord apparently does not function in light reception. 



Numerous gonads, which are either all testes or all ovaries, for the 

 sexes are separate in Amphioxus, bulge into the atrial cavity. Actually, 

 they lie within a portion of a highly modified coelom (Fig. 20.4). The 



