Veliger: A larval stage of a mollusk characterized by the presence of a velum.

Velum: A ciliated swimming organ that develops in certain larval stages of most marine gastropod mollusks. A covering or partition of thin membranous tissue, such as the veil of a mushroom or a membrane of the brain.
http://www.reefs.org/library/glossary.html#V


Part 9- Phylum MOLLUSCA
Mollusks are one of the mega-sized phyla in the ocean. The name Mollusca comes from the Greek, mollis, which means
“soft,” and all members of this phylum have soft, muscular bodies that are usually protected by one or more external
calcium shells (in adulthood). The shell forms early in life and helps us determine which mollusk class a planktonic larva
belongs to. Gastropod larvae have a single, spiral shell and Bivalve larvae have two “clam shaped” shells. Mollusks
undergo at least two stages in the plankton before their final metamorphosis to a benthic life. The first stage of mollusk
development is known as the trochophore larva and is very similar to the Polychaetes stage of the same name. The
second stage is the veliger larva from the latin word velum, which means "veil" or "wing." The velum is a large beautiful
winged structure bordered by actively beating cilia. Veligers live in the plankton and grow until their shells become too
big and heavy to float. They sink as they undergo a final metamorphosis to a benthic adulthood.
A Guide to the
M A R I N E P L A N K T O N
of southern California
This Guide, and other resources, are available for download a no cost from:
http://www.msc.ucla.edu/oceanglobe
and is intended for non-profit, face-to-face, educational use ONLY.
© 2003 Robert Perry. All Rights Reserved.
name________________________________________________________
Robert Perry - UCLA OceanGLOBE & Malibu High School

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Note added at 1 hr (2007-09-03 13:49:03 GMT)
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I chose gastropode over bivalve because of your text saying يزحف usually in arabic gastropode is translated by بطنيات الأقدام (معجم أحمد شفيق الخطيب)