Pycnogonida developmental biology
2025
John A. Fornshell
The term anamorphic development is used to describe arthropods which add segments throughout their lives. Epimorphic development is used to describe arthropods which hatch with a set number of segments which does not increase with subsequent molts. Some arthropods add segments initially until a set number have been produced after which no further segments are produced with subsequent molts. The latter are said to have hemianamorphic development. In the Chelicerata the completion of embryonic development and hatching of the egg are not as intricately linked as in the other classes of the Arthropoda. Thus, making the distinction of which developmental pattern is occurring less obvious. The members of the Pycnogonida typically hatch as a "Protonymphon larva" having three pairs of appendages and a proboscis with a tripartite mouth. The subsequent growth of the animals displays certain developmental processes typically seen in embryos: (1) Apoptosis; (2) invagination of ectodermal tissues to initiate the formation of neurogenic niches. (3) organogenesis; and (4) limb development progressing in an anterior-posterior developmental gradient. If the "Protonymphon larva" and early nymphs are interpreted as embryonic stages, then the Pycnogonida may be considered to have epimorphic development.
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