Veliger

The general structure of the veliger includes a shell that surrounds the visceral organs of the larva (e.g., digestive tract, much of the nervous system, excretory organs) and a ciliated velum that extends beyond the shell as a single or multi-lobed structure used for swimming and particulate food collection: veliger signifies "velum bearer."

[1] Metamorphosis of feeding and non-feeding competent larvae is usually induced by a chemical cue characteristic of the proper habitat for the juvenile.

In bivalves, the chemical cue may be produced by bacteria specific to the type of biofilm growing in the adult habitat.

This sudden rotation of the bodily organs relative to the rest of the animal may take anywhere from three minutes to ten days, depending on species.

In many species, induction of metamorphosis occurs as a sensory response to a chemical cue indicative of the juvenile and/or adult habitat.

As this occurs, the shell (known as a prodissoconch) and structures such as the larval foot, velum and visceral organs increase in size.

As is the case for planktotrophic gastropod veligers, the larvae continue to feed and grow until they develop the organs and systems necessary for metamorphosis to the juvenile stage.

During metamorphosis, the veliger sheds its velum and, depending on species, may secrete an attachment structure called a byssus that anchors it to the substratum.

Some species spend considerable time searching for an ideal habitat before metamorphosing, but others may settle on the nearest suitable substrate.

[2] The scaphopods, or tusk shells, have a veliger larva very similar to that of bivalves, despite the great difference in the appearance of the adults.

However, unlike bivalves, this never splits into two, and, in fact, fuses along the ventral margin, eventually becoming a tube that encloses the length of the body, and is open at both ends.

The scaphopod veliger is free-living, and metamorphosis is marked by a great elongation of the body, in order to assume the adult form.