EYES OF INVERTEBRATES 83 



equilibrating organs are found in Hormiphora plumosa, one of the comb- 

 bearing free-swimming coelenterates. The class Ctenophora to which 

 this species belongs is, moreover, of special interest in tracing the history 

 of the sense-organs owing to the occurrence in these animals of a definite 

 bilateral symmetry, as indicated by right and left tentacles and more 

 especially, as in Hydroctena, by the presence of an ampulla at the apex 

 containing two lithites supported on spring-like epithelial processes. 

 Pigment spots are arranged as in the more primitive coelenterates circum- 

 ferentially round the body. Moreover, the development of a definite 

 sub-epithelial plexus of nerves in connection with the sense-organs beneath 

 the general surface of the body and in the tentacles is worthy of note, as 

 this sup-epithelial plexus of nerve-fibres and cells is the precursor of 

 the central nervous system of the higher types of invertebrate animals 

 (Fig. 53), and it is generally supposed that the central nervous system of 

 the prevertebrate ancestors of the vertebrates arose in the same way. 



