366 CEPHALOCHORDATA. 



{d) The " fin-rays," the sheath which extends from the 

 notochord around the nerve-cord, the septa of connective 

 tissue between the muscle-segments may also be included. 

 To the latter the < shaped markings of the animal are due. 



Muscular System. — The animal swims by wriggling its 

 body laterally. This is effected by the lateral muscles, 

 which are divided into muscle-segments, myotomes, or 

 myomeres. In these the muscle-fibres run longitudinally, 

 but there is a transverse set on the ventral surface between 

 the mouth and the atriopore. They help to drive the water 

 out of the atrial cavity. Other muscles occur about the 

 mouth, etc. Nearly all the fibres aj-e striated. 



Nervous System. — The spinal cord, lying within a con- 

 tinuation of the notochord-sheath, extends along the mid- 

 dorsal line. 



From the cord two independent sets of nerves arise, 

 dorsal nerves corresponding to the segments, ventral nerves 

 much more numerous. These two sets are comparable to 

 the single-rooted, sensory, dorsal nerves, and the many- 

 rooted, motor, ventral nerves of higher Vertebrates ; but 

 in Amphioxus the two sets do not unite. Moreover, the 

 dorsal nerves pass to the muscles as well as to the skin, 

 and they have no ganglia. Nor are there any sympathetic 

 ganglia. 



There is no anterior swelling or brain. It seems, how- 

 ever, that there is some histological distinctiveness about the 

 anterior region, and that the first five pairs of sensory nerves 

 are to be regarded as cerebral. 



With the anterior region, where a brain ought to be, the 

 following structures are connected : — 



(a) On the left side, there is an olfactory (?) ciliated pit, 

 connected by a tube with the canal of the nerve-cord. It 

 may be that this is a "neuropore" or aperture left when 

 the folds of the medullary groove closed to form a canal. 



{b) There is also a pigment-spot, sometimes called an 

 eye-spot, at the very end of the nerve-cord. 



(c) A sac, apparently containing sensitive cells, opens on 

 the roof of the mouth. It may be a tasting or smelling 

 organ. It arises from the left of two pouches, which grow 

 out anteriorly from the front of the gut or archenteron of 

 the embryo. 



