The most useful generalization to emerge from modern studies is the recognition that primary marine larvae can be separated into two basic types, depending on whether they are organized in a downstream or an upstream fashion. The terms refer to the position of the main ciliary band and the direction of ciliary beat in relation to the mouth. In downstream larvae, the mouth is located behind (posterior to) the principal band, whose cilia beat towards it. This means that locomotory and feeding currents coincide. The larvae can thus feed as they swim, and are typically rather swimmers with a compact body.
In upstream larvae, in contrast, the mouth is anterior to the main locomotory ciliary bands, and the latter beat away from the mouth. Water currents generated during locomotion cannot then be used directly for food capture. Instead, a variety of indirect means of concentrating food particles are employed. These appear to depend largely on hydrodynamic effects generated by the complex topology of the body surface [specific ciliary behaviors, e.g. reversals, may also be involved in some instances]. In consequence, upstream larvae are often large, with large surface areas, swim slowly, and have surprisingly bizarre shapes. They are also usually transparent and rather delicate, in part because rapid growth to comparatively large size, using minimal amounts of tissue, evidently confers a significant adaptive advantage. Upstram larvae consequently make excellent microscopical subjects, and it is no accident that they figure more prominently than downstream larvae in research on the mechanics of development whenever direct observations on live larvae are required.
Phylogenetic trees constructed on the basis of larval type accord well with those based on other criteria, e.g. molecular sequences. Two major, well-defined groups emerge, protostomes and deuterostomes, the former with downstream larvae and the latter with upstream ones. Larval morphology thus provides additional support for the premise that the protostome/deuterostome distinction is a natural one of long evolutionary standing.
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