  In order to understand how animals evolved over time, biologists must learn
how their body parts form during their development. The following is a detailed
description of how one species of sea spider transforms from a hatchling to an
8-legged adult. Pycnogonids, or sea spiders, comprise a primitive lineage of
arthropods. As such, they hold potential to reveal insights into arthropod
evolution. Recent phylogenetic analyses have supported their position as either
basal chelicerates or as a separate, fifth major lineage of extant arthropods.
Disagreements concerning pycnogonid relations to other arthropods is partly a
product of too few primary observations of pycnogonid anatomy and development.
This investigation of post-embryonic development of the pycnogonid
Anoplodactylus eroticus employs multiple techniques of anatomical observation,
including Nomarski optics, scanning electron microscopy and fluorescence
microscopy, in order to thoroughly document the life cycle.
  After the second post-embryonic stage, larvae of A. eroticus burrow within a
hydroid and undergo morphogenesis. Larvae emerge from the hydroid and
simultaneously molt into the juvenile stage. Over the course of post-embryonic
development there are eight stages preceding the mature adult. All structures,
except for the anteriormost appendages, the chelifores, undergo some degree of
transformation. Chelifores are present prior to hatching and remain mobile over
the course of development. Some larger issues important to arthropod evolution
are addressed, such as the equivalent of a germband and labrum in pycnogonids.
Post-embryonic development of A. eroticus provides an example counteracting
previous reports of anamorphic development and a four-segmented head in the
pycnogonid ground pattern, findings that were extrapolated to fit the ground
pattern of Arthropoda.
