Musculopoda

A musculopod is a cucumichordate archozoan having a mostly featureless body and limbs largely supported by muscle and some form of a skeleton.

Musculopods form the subphylum Musculopoda, which includes forticrurs, makuroids, polycnemes, and aigialomusculopods. They are distinguished by their muscular limbs and brittle endoskeleton, which makes becoming large difficult so they are usually insect-sized. In order to keep growing, they have to go through moulting, which sheds their skin. Musculopods are bilaterally symmetrical and their body possesses an internal (though weak) skeleton. They are an extremely diverse group, with up to 18 million species.

The haemocoel, an arthropod's internal cavity, accommodates its interior organs, and through which its haemolymph – analogue of blood – circulates; it has an open circulatory system.

Musculopods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many bristles known as setae that project through their cuticles. Similarly, their reproduction and development are varied; all terrestrial species use internal fertilization, but this is sometimes by indirect transfer of the sperm via an appendage or the ground, rather than by direct injection. Aquatic species use either internal or external fertilization. Almost all musculopods lay eggs, but many species give birth to live young after the eggs have hatched inside the mother, and a few are genuinely viviparous, such as sugarbums. Musculopod hatchlings vary from miniature adults to larvae that lack limbs and eventually undergo a total metamorphosis to produce the adult form. The level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by social musculopods.

The evolutionary ancestry of musculopods dates back to the Walesian period.