ii. ii RECEPTORS OF AMPHIOXUS 41 



been attempts to show that this represents an eye. More probably 

 it serves to prevent rather than to receive photic stimulation; there 

 are other cells lying in the spinal cord that are clearly photoreceptors 

 (Fig. 9). In the front part of the body these are unprotected by pig- 

 ment, whereas more posteriorly they are so pigmented as to be pro- 

 tected asymmetrically from the light (Fig. 16). This asymmetry may 

 be connected with the fact that when swimming free in the water 

 amphioxus moves spirally about its axis, turning clockwise as seen 

 from behind. It was established by Parker that a small beam of light 

 produces movements of amphioxus only when it is directed on to the 

 region of the body or tail, not when it shines on the head. Since the 

 animal normally lies with the head protruding we may suppose that 

 the pigment spot serves to prevent light that strikes down vertically 

 from stimulating the photoreceptors in the cord. 



Amphioxus is therefore provided with receptor and motor systems 

 that serve to keep it in its sedentary position, able to collect food from 

 the current that it makes by the cilia (p. 33). There are mechanisms 

 that help it to make appropriate movements of escape when it is 

 touched or when the body (but not head) is illuminated. The touch 

 receptors of the buccal cirri produce rejection of large particles and 

 those of the velum are chemo-receptors. The infundibular organ may 

 be some form of gravity or pressure receptor. By means of these 

 receptor organs and its simple movements of swimming, burrowing, 

 and closing the oral hood, the animal is maintained, probably mainly 

 by trial and error (phobotactic) behaviour, in an environment suitable 

 for its life. There are none of those elaborate mechanisms that we find 

 in higher chordates for 'seeking' special environments or for so 

 'handling' or managing them that they may prove habitable by the 

 animal. Amphioxus must take and leave the world very much as it 

 finds it. The 'correct' environment is chosen for it by the selective 

 settling of the larvae. 



1 1 . Gonads and development of amphioxus 



The gonads of amphioxus are hollow segmental sacs with no com- 

 mon duct. Each sac develops from mesoderm cells, perhaps originally 

 from a single cell, at the base of the myotomes in the branchial region, 

 the genital cells themselves developing on the walls (Fig. 6). The sexes 

 are separate and the genital products are shed by dehiscence into the 

 atrium, the aperture by which they escape closing and the gonad 

 developing afresh. 



Extrusion of the gametes occurs in spring, on warm evenings 



