EXPERIMENTAL PRODUCTION OF TWINS 17 



Another common type of twin larva is one in which 

 there seems to have been a partial obliteration of the 

 primary symmetry relations and two secondary equiv- 

 alent symmetries result. In such larvae both archen- 

 tera develop symmetrically and are quite equivalent, 

 so that it would not be possible to call one the 



Figs. 1-3. — Outline drawings of three types of twin larvae of the 

 starfish Patiria miniata, showing plural archentera. Fig. i, an advanced 

 bipennaria with primary archenteron at the original basal pole and a well- 

 developed secondary archenteron at the site of the original apical pole. 

 Fig. 2, a similar bipennaria with two supernumerary archentera, a 

 secondary and a tertiary. Fig. 3, a true twin larva with two sym- 

 metrically placed archentera, neither of which is the primary one. 

 (From Newman.) 



