GENERAL FEATURES 459 



the thickest. There is practically no brain, though the 

 two pairs of nerves which pass to the sensitive front and 

 above the mouth are different from the other nerves and may 

 be called cerebral. There are many peculiarities in the 

 spinal cord : thus there are two pairs of spinal nerves 

 (instead of the normal one pair) for each myotome, there are 

 no spinal ganglia or sympathetic ganglia. There is no 

 brain-eye; what is sometimes called an "eye-spot" in front of 

 the end of the nerve-cord being only a pigment spot. 

 Another remarkable peculiarity is the occurrence of " spinal 

 eyes " at regular intervals right down the nerve cord, beside 

 and below the central canal. Each consists of two cells — a 

 pigment cell and a percipient cell. They seem to be sensitive 

 to light which passes through the translucent body. 



(2) The notochord is a solid elastic rod with a thin sheath 

 of connective tissue, but it is not even at the level of cartilage 

 in the strict sense. It consists of vacuolated cells, and its sup- 

 porting power is probably in great part due to their turgidity, 

 as in many vegetable structures. Another quite unique peculi- 

 arity is that the notochord is prolonged beyond the end of the 

 nerve-cord to the very tip of the body. 



(3) The gill-slits are numerous, sixty pairs or so, while in 

 fishes they do not exceed eight pairs. Moreover, the number 

 is added to posteriorly during the adult life of the Lancelet. 

 Primarily, the gill-slits open directly to the exterior, but they 

 soon come to open into a special chamber — the atrial chamber 

 — which communicates with the exterior by the atriopore. 



Thus we can define the Cephalochorda as follows. 



There is a tubular nerve-cord along the dorsal or blastoporal 

 surface, arising as an ectodermal median groove which becomes 

 a canal. It shows in the embryo a temporary posterior con- 

 nection with the gut-cavity called the neurenteric canal. There 

 is hardly any brain, though the first two pairs of nerves may be. 

 called cerebral. The dorsal and ventral roots of the spinal 

 nerves do not unite. There are no brain-eyes nor auditory 

 organs. 



The notochord, which arises as an axial differentiation of 

 cells along the median dorsal line of the primitive gut or 

 archenteron, is persistent and unsegmented. It extends to the 

 very anterior tip of the body, beyond the front of the nerve- 



