AMPHIOXUS 17 



the spot where the cavity of the tube opened to the exterior 

 at earlier stages (the neuropore). 



The cavity of the nerve-tube is enlarged at its front end 

 forming the cerebral vesicle. At the same time the external 

 diameter of the tube remains the same ; its walls are here 

 therefore thinner. This specialisation is in Amphioxus the only 

 indication of a brain. At the front end of the nerve-tube is a 

 pigment-spot, to be regarded as a visual organ. Other such 

 pigment-spots, or primitive " eyes," are to be found further 

 back near the central canal. 



On each side of the body the nerve-cord gives off nerves, 

 which are of two kinds, dorsal and ventral. In each segment 

 on each side of the body there is one dorsal nerve-root and 

 one bunch of ventral nerve-roots. The ventral roots are 

 distributed solely to the muscle-fibres in the myotome of that 

 segment, and are " motor " nerves. The dorsal roots are 

 concerned with transmitting impulses received from the 

 sense-organs all over the skin (especially numerous on the 

 buccal cirrhi), and with innervating the smooth musculature 

 of the gut and atrium. The axons which go to make up the 

 sensory or afferent fibres of the dorsal nerve-roots are derived 

 directly from the sensory cells in the skin. The sensory cells 

 therefore convey their impulses direct to the central nervous 

 system on the plan characteristic of many invertebrates. In 

 all forms above Amphioxus, the impulses are collected from 

 the sensory cells by axons derived from other nerve-cells, 

 whose nuclei lie in swellings or ganglia on the dorsal nerve- 

 roots. Amphioxus is therefore primitive in not possessing 

 these ganglia or nerve- cells. 



The most anterior two pairs of roots are dorsal, and have 

 no ventral roots corresponding to them. They innervate the 

 sense-organs of the snout, oral hood, and buccal cirrhi. 



While reviewing the foregoing description of Amphioxus 

 an important analysis can be made. In the light of knowledge 

 of other forms, the characteristics of an animal can be divided 

 into two classes : primitive and specialised. There are those 

 characters which are developed and perfected in the process 

 of evolution to the next stage, and which are therefore simpler 



C 



