THE LOWER VBRTEBRATA. 203 



olfactory epithelium. These cells are probably sensitive to 

 various kinds of stimuli, but principally to touch. On 

 the left side is a funnel, richly ciliated, leading down to the 

 front end of the brain ; this is very possibly an organ of 

 "smell" or " taste," i.e. one which recognizes any unpleasant 

 material that there may be in the water, and sends a 

 stimulus into the brain which sends out a reflex motor 

 impulse to the sphincter of the mouth, to close the^aperture 

 until the objectionable substance is gone. 



Though it has no eyes and cannot see distinct objects, 

 Amphioxus can distinguish between light and darkness, and 

 doubtless has an instinct to burrow if a large shadow falls 

 on it a warning of some predaceous fish. This sensitive- 

 ness to light appears to reside in the epithelium of the 

 central canal of the spinal cord, for irregularly shaped 

 masses of pigment occur in it at close and regular intervals. 

 A similar mass occurs in the epithelium at the very front end 

 of the brain, and is usually called the " eye-spot" a term 

 equally applicable to the others. The existence of a sensitive 

 epithelium inside the spinal cord is very remarkable, as it 

 is usually the external surface which is sensitive. We 

 shall return to this point in dealing with the development 

 of the vertebrate eye. Of course, such a condition is only 

 possible in a translucent animal like Amphioxus. 



The simplicity of the central nervous system can be easily 

 correlated with the other conditions in Amphioxus. There 

 being on the one hand no elaborate sense-organs con- 

 tinually sending sensory impulses in, and on the other hand 

 no elaborate muscular system for breathing, eating, the 

 more complex kinds of locomotion, etc., there is need only 

 of a simple co-ordinating system. 



13. The Excretory System of Amphioxus differs con- 

 siderably from that of our other types. There is no com- 

 pact kidney. Instead we have a metameric series of quite 

 separate tubules one to each primary gill-slit. Each of 

 these tubules is bent on itself nearly at right angles, and 

 opens at one end into the atrium. 



These excretory tubules were formerly supposed to have 

 several openings into the dorsal coeloinic canal and to be 



