THE EYES OF HEMICHORDA 



173 



plate and eyes atrophy and the larva sinks to the bottom and undergoes 

 a metamorphosis whereby it is changed to a worm-like form which leads 

 a burrowing life, like Amphioxus, and has no eyes. 



A B 



Fig. 126. — Cephalodiscus. (After McIntosh and Harmer.) 

 Colony showing gelatinous investment covering separate zooids. B 



Dia- 



Cephalodiscus is closely related to Balanoglossus, but there is no free-swimming 

 Tornaria, having eye-spots. 



Cephalodiscus (Fig. 126) and Rhabdopleura, which lead an inactive, 

 fixed life, being associated in colonies, have no eyes in the mature condition, 

 but Mcintosh has described eye-spots as being present in the polypides of 

 Cephalodiscus. 



