Not all invertebrate phyla with marine representatives have pelagic larvae. Many, including those specialized for small body size, e.g. most ecdysozoans, have direct development, whereby the hatching stage is simply a small version of the adult. Among the invertebrates that have larvae, there is usuaully a well-defined type larva for each phylum. Each type is presumably primitive larval type for the phylum, and similarities between them can be usd in some instances to link them in a phylogenetic series. How far back in time such series can be taken is a matter of debate. Jägersten, at one extreme, has postulated an ancestral pelagobenthic life cycle (pelagic larva + benthic adult) for metazoans as a group, which would makes pelagic larvae very ancient indeed. Larvae descended directly from the ancestral type, assumed to resemble the planula of modern cnidarians, are referred to as primary larvae. The majority of pelagic marine invertebrate larvae (i.e. those considered below) fall into this category. A number of secondarily evolved larvae (secondary larvae) also occur among invertebrates. These are generally modified juvenile stages of what were originally direct-developing groups, and are seen most notably in arthropods, parasitic flatworms and chordates.
Much of the interest in invertebrate larvae stems from the assumption that the primary larva concept is correct to at least a degree. This would mean that a good deal of phylogenetically useful information can probably be extracted from modern larvae by judicious application of morphological and molecular techniques, and this is currently an active area of investigation.
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