>Are there species with the veliger phase and
>species without it in the same Genus ?

Yes, it's a common occurrence in the Nassariidae, Costellariidae,
Rissoidae, Olividae and many other groups.

>Can we consider a mollusk with a veliger phase
>more evolved than a mullusk without it ?

>If so, is it correct to consider the species
>without veliger more primitive than the other ones
>( inside the same Genus ) ?

No, because species without the veliger phase generally evolve from species
with the veliger phase. It is quite easy to lose the veliger stage, but
difficult to impossible to re-evolve it, depending on how it was lost. This
means that the veliger stage is usually primitive (appearing earlier in the
history of a lineage) and lack of the veliger stage is derived (occurring
later in the history of the lineage).

Gary

---------------------------------------------------------------------
Gary Rosenberg, Ph.D.                     [log in to unmask]
Malacology & Invertebrate Paleontology    gopher://erato.acnatsci.org
Academy of Natural Sciences               http://www.acnatsci.org
1900 Benjamin Franklin Parkway            Phone 215-299-1033
Philadelphia, PA 19103-1195 USA           Fax   215-299-1170