THE EYES OF AMPHINEURA 157 



situated one behind another on the back of the animal. These give 

 an appearance of segmentation. This is, however, misleading, as the 

 body itself is not truly segmented. Each plate or valve is developed as 

 a separate scale, the scales later uniting to form a jointed shield or lorica. 



Fig. 115. 



Dorsal aspect of Chiton spinosus, a mollusc in which the head of the adult animal 

 has no eyes, tentacles, or statocysts. Sensory organs are, however, found in 

 canals which are present in the superficial layer of the shell valves which 

 cover the back. Some of the larger of these organs have the structure of an 

 eye, having a cornea, lens, iris, pigment layer, and retina. 



(From Cambridge Natural History.) 



From beneath the edges of the lorica bristles project, which, diverging 

 from one another in all directions, give a brush-like appearance to the 

 animal. The superficial cuticular layer of the scales is perforated by small 

 vertical canals which lodge sense-organs. These are called " micraesthetes" 

 and " megalaesthetes," and some of the latter are especially differentiated 

 as eyes, having a cornea, lens, sensory retinal cells, a pigment layer, and 

 iris. 



Ocelli are also present in the trochophore larva (Fig. 116). 



The arrangement and structure of the tubular system and sensory 



