{127}The CENTRAL NERVOUS SYSTEM is dorsal and tubular as in Vertebrates, and lies in a connective-tissue sheath immediately above the notochord (Figs. 71, etc., and 80, A). Posteriorly it tapers to a fine point a little in front of the end of the notochord, but anteriorly it ends abruptly some distance behind the anterior extremity of the notochord. The central canal is connected with the dorsal surface by a median longitudinal cleft (Fig. 80, C), and at the anterior end it dilates to form the cerebral vesicle (_c.v_) with which two simple sense-organs, an eye-spot (_e_) and an olfactory pit (_olf_), are connected. A patch of ciliated epithelium in the floor of the vesicle has been described as an "infundibular-organ." There is also a surface dilatation of the dorsal cleft behind the cerebral vesicle (_dil_). The nervous system as far back as this point may be regarded as the brain, though scarcely distinguishable externally (Figs. 71 and 80, A) from the spinal chord behind. From this "brain" arise two pairs of "cranial" nerves, the first (I.) from the anterior end, and the second (II.) from the dorsal surface of the cerebral vesicle; both are in front of the first myotomes of the body, and supply the pre-oral snout with nerves. [Illustration: FIG. 79.—Nephridia. A, portion of a nephridium of _Phyllodoce_, a marine Polychaete, for comparison with B, portion of a nephridium of Amphioxus. These figures show the solenocytes with their flagella projecting through the long tubes into the lumen of the excretory organ, and demonstrate the essential similarity of the nephridia of Amphioxus with those of Polychaete worms (after Goodrich).] The spinal cord gives off a large number of spinal nerves segmentally arranged, but, like the myotomes, not opposite and symmetrical on the two sides, but placed alternately (Fig. 81). Moreover, the spinal nerves arise on each side at two levels, there being a more dorsal series each arising by a single root and {128}supplying the integument as well as the transverse muscles, so as to be sensory as well as motor, and a ventral series arising each by a number of roots (Fig. 81) and wholly motor in function, as they supply only the myotomes. These two series may be compared to the dorsal and ventral roots which in the Vertebrata join to form a mixed spinal nerve. [Illustration: FIG. 80.—_Branchiostoma lanceolatum_. A, brain and cerebral nerves of a young specimen; B, transverse section through neuropore; C, behind cerebral vesicle; D through dorsal dilatation. _ch_, Notochord; _cv_, cerebral vesicle; _dil_, dorsal dilatation; _e_, eye-spot; _np_, neuropore; _olf_, olfactory pit; _I_ and _II_, cranial nerves. (From Willey, after Hatschek.)] In addition to ordinary small nerve cells the central nervous system contains certain large nerve cells with very long processes, the "giant fibres," which extend through the greater part of the length of the spinal cord. No trace of a sympathetic nervous system has been found.