478 TUNICATES 



beside the oviduct there is a vas deferens for liberating the 

 spermatozoa developed in the testis. In some Ascidians, such as 

 Ascidia mentula, the eggs are fertilised by the sperms from the 

 same animals, but in many cases the two sets of reproductive 

 organs are not ripe at the same time (dichogamy) so that self- 

 fertilisation is prevented. In some the ovary ripens first 

 (protogynous dichogamy) ; in others the testis ripens first 

 (protandrous dichogamy). 



