Within the classification of life, the phylum Tardigrada is
closely associated with the Onychophora and Arthropoda forming
the Panarthropoda. In growth, tardigrades shed their cuticle—
ecdysis—and are grouped with other similarly molting invertebrate
animals under the title Ecdysozoa. There are two classes within the
phylum Tardigrada; the Heterotardigrada, distinguished by cephalic
sensory cirri and including most of the marine species and terrestrial
forms with “plated” cuticles; and the Eutardigrada, lacking
cephalic sensory cirri are mostly limno-terrestrial, “naked” forms.