This brief account summarizes some basic features of marine invertebrate larvae, with remarks on their phylogenetic significance. It is intended for non-specialists or students new to the subject. The remarkable diversity of form among marine larvae was first appreciated in the mid-19th century, when they were discovered in plankton by Johannes Müller and his students. The larvae illustrate the tendency of evolution to produce variations on the theme in an especially vivid fashion, and as such, they appear prominently in the writings of evolutionary biologists during the latter half of the century. We are still uncertain as to the precise evolutionary relationship between larvae of different phyla. In recent years, major contributions to our understanding of larval diversity and evolution have come especially from Scandinavian biologists, notably Gosta Jägersten, Gunnar Thorson and, most recently, Claus Nielsen (see references).
According to current thinking, the diversity of form among marine larvae largely reflects the specific adaptations required for planktonic feeding and locomotion, but this occurs against a background of evolutionary constraints that arise from the inherited ground plan and developmental sequence. It remains a major task to separate these two components, so that specific and largely secondary specializations can be distinguished from the basic features on which a phylogenetic sequence can eventually be built.
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