388 



HITCHjCOCK'S anatomy 



minths are without eyes. Among Arachnida (spiders) the 

 eyes bear a near relation to the vertebrate type of eyes. Their 

 number is much less than among the other articulata, seldom 

 more than eight, and are to be compared more with stemmata 

 than with compound eyes. Sometimes these are collected 

 into one mass upon the second segment of the body, and some- 

 times they are arranged symmetrically upon the median line. 



712. Eyes of Molluscs.— The organs of vision in the 

 Acephala are numerous, rising as high as forty in the genus 

 Pinna, where they are placed in the mantle. The Cephalo- 

 phora have generally two eyes. In the Cephalopoda the eyes 

 are disproportionately large, and possess nearly all the parts 

 found in the eyes of vertebrates. 



713. Eyes of Radiates. — The Polypi show a sensibility 

 to light, but no eyes have been discovered in them. The 

 Acalephae seem to have the sense of vision, and Agassiz seems 

 to have ascertained the presence of an organ for this purpose 

 in some of them. It is more doubtful in respect to the Echi- 

 noderms. 



What eyes have spiders? 712. What are the eyes of molluscs? T13. What is said #f 

 the organs of vision among radiates ? 



