CHAPTER l6 



CEPHALOCHORDA— ACRANIATA 



The pigment cells and light-percipient organs found in the lancelet 

 fish, or Amphioxus, have been specially studied by Hesse and Joseph ; 

 the latter has shown that the pigment spot beneath the unpaired olfactory 

 lobe, which has been described as the eye spot or median eye of the 

 lancelet, consists merely of a cluster of cylindrical cells, filled with pig- 

 ment granules which occupy the extreme anterior end of the " so-called " 

 cerebral ventricle, and that in his opinion there is no structural resem- 

 blance between this group of pigment cells and even the simplest type 

 of vertebrate eye. He also described similar cells in the dorsal region 

 of the anterior part of the spinal cord. Neither the cells of the pigment 

 spot or of the larger group at the anterior end of the spinal cord (cells 

 of Joseph) are in relation with neuro-sensory cells, and it has been 

 proved experimentally that neither are sensitive to light. Moreover, 

 if these regions are removed by decapitation, the body of the animal still 

 reacts to light. The explanation of this lies in the existence of the organs 

 of Hesse (Fig. 129, A and B, p. 179), which consist of a series of 

 single large neuro-sensory cells capped by a crescentic pigment layer 

 which covers over one-half of the organ ; a nerve process arises from 

 the sensory cell, either from beneath the edge of the pigment cap, or 

 opposite the centre of the cap. The pigment is arranged in three distinct 

 layers, a thick band in the middle and two thin layers external and internal 

 situated respectively superficial and deep to the middle layer. Between 

 the pigment layers are two unpigmented strata which are believed by 

 Boeke to contain neurofibrillar in the cell-protoplasm. 



The arrangement of the organs of Hesse in the spinal cord is 

 remarkable. They are situated ventral and lateral to the central canal 

 of the spinal cord commencing at the third or fourth segment, where 

 they are most numerous and becoming less frequent towards the tail 

 end. It is said also that on the left side of the cord the " eyes " are 

 directed upward or dorsalward ; on the right side, downward or ventral- 

 ward (Fig. 129, A). Kappers remarks that as the animal usually lies on 

 one side it would in this position perceive light reaching it in a horizontal 

 plane. Beside the organs of Hesse, other neuro-sensory cells are found 



178 



