There is a review of this issue in a paper I did last year.
Trochophore concepts: ciliary bands and the evolution of larvae in
Biological Journal of the Linnean Society 66(4):411-464.
ABSTRACT: 'Trochophore' is a term used in a strict sense for larvae having
an opposed-band method of feeding, involving a prototroch and metatroch.
Other ciliary bands such as a telotroch and neurotroch may be present.
The trochophore has been proposed to represent the ancestral larval form
for a group of metazoan phyla (including all members of the Spiralia). The
name trochophore is also often applied to larvae that do not conform to the
above definition. A cladistic analysis of spiralian taxa (with special
reference to polychaete annelids), based on a suite of adult and larval
characters, is used to assess several hypotheses: (1) That the trochophore
(in a strict sense) is a plesiomorphic form for the Spiralia; (2) That the
strictly defined trochophore is plesiomorphic for members of the Spiralia
such as the Polychaeta. The homology of each of the various separate
ciliary bands of spiralian larvae, and features such as the apical tuft and
protonephridia is also assessed. The results favour the conclusion that the
trochophore, if defined as a feeding larval form using opposed bands,
should not be regarded as an ancestral (= plesiomorphic) type for the
Spiralia, or any other large taxon such as the Polychaeta or Mollusca. The
trochophore is re-defined as a larval form with a prototroch and
plesiomorphic features such as an apical plate and protonephridia. This
broad definition covers a wide variety of larvae, and matches the current
usage more accurately than the restricted term. Features such as the
neurotroch, telotroch and opposed-band feeding show convergence and
reversals. The nature of the metatroch requires further investigation. The
presence of a prototroch (and hence a trochophore larvae) is used to
identify an apomorphy-based taxon, Trochozoa, that includes the first
ancestor to have evolved a prototroch and all its descendants. This
minimally includes the Annelida (sensu lato), Echiura, Entoprocta, Mollusca
and Sipuncula and is a less inclusive taxon than the Spiralia.
As for how to subdivide the various forms of trochophores among
polychaetes, consensus is yet to be reached.
School of Biological Sciences A08
University of Sydney
gregr at bio.usyd.edu.au
Tel. (02) 9351 5571
Fax (02) 9351 4119
International: 61 2 replaces 02
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