II. IO 



RECEPTORS OF AMPHIOXUS 



39 



Ten Cate has investigated the movements of amphioxus and found 

 that it responds to all stimuli by movements of 'flight'. There are no 

 isolated or local movements; the effect of any stimulus such as touch 

 on the side of the body is to produce waves of myotomal contraction. 

 These may, however, vary from strong waves going the whole length 



g-ep. 



s.ep. 



p.sp. 



B 



n.pr. o.c. t.po.h 



Fig. 15. Diagram of (a) the anterior end of the nervous 

 system of amphioxus and (b) the brain of a fish 

 (Polypterus). 



A. Amphioxus. g.ep. granulated ependyma in the wall of the 'dorsal 

 central canal'; i.o. infundibular organ; p.sp. pigment spot; r.f. 

 Reissner's fibre in the central canal; s.ep. sensory epithelium, 

 u. Polypterus, a.c. anterior commissure; aq.s. aqueductus Sylvii; cer. 

 cerebellum; ep. epiphysis; m. medulla spinalis; n.h. neurohypophysis; 

 n.pr. nucleus praeopticus; o.c. optic chiasma; r.f. Reissner's fibre in 

 fourth ventricle; s.c.o. subcommissural organ; s.d. saccus dorsalis; 

 s.v. saccus vasculosus with primary sense cells; t.po.h. tractus praeop- 

 tico-hypophyseus. (After Olson and Wingstrand.) 



of the body to single rapid twitches. The giant cells participate in the 

 spread of these waves. It seems likely that the arrangement ensures 

 that touch on the anterior part of the body, normally exposed when 

 feeding, produces backward movement (i.e. withdrawal into the sand) 

 but touch on the hind part the reverse movement of emergence and 

 escape. 



At the front end the central canal is enlarged to form a cerebral 

 vesicle (Fig. 14). The whole neural tube is hardly wider here than in 

 the region of the spinal cord and there is no thickening of the walls, 



