380 PRIMITIVE RADIAL SYMMETRY OF ECHINODERMATA. 



The absence of sense organs on the praeoral lobe of larval 

 Echinodermata, coupled with the structure of the nervous system 

 of the adult, points to the conclusion that the adult Echinoder- 



FlG. 232. A. PlLIDIUM WITH AN ADVANCED NEMERTINE WORM. B. RlPE 

 EMBRYO OF NEMERTES IN THE POSITION IT OCCUPIES IN PlLIDIUM. (Both after 



Butschli.) 



a. oesophagus ; st. stomach ; i. intestine ; pr. proboscis ; lp. lateral pit (cephalic 

 sack) ; an. amnion ; . nervous system. 



mata have retained, and not, as is now usually held, secondarily 

 acquired, their radial symmetry; and if this is admitted it follows 

 that the obvious bilateral symmetry of Echinoderm larvae is a 

 secondary character. 



The bilateral symmetry of many Ccelenterate larvae (the 

 larva of ^Eginopsis, of many Acraspeda, of Actinia, &c.), coupled 

 with the fact that a bilateral symmetry is obviously advanta- 



