For limnoterrestrial
eutardigrades fertilization usually occurs inside the
body, while some marine heterotardigrades have seminal receptacles,
and fertilization is external. Eggs can be grouped into four
types: smooth eggs deposited directly into the substrate (marine
heterotardigrades), sculptured eggs deposited in the substrate [terrestrial
and freshwater eutardigrades], smooth eggs deposited in
the cast cuticle (terrestrial heterotardigrades, marine eutardigrades,
some freshwater and terrestrial eutardigrades), and smooth eggs
deposited in the cast cuticle which is then carried (this limited
parental care has been observed in a few freshwater eutardigrades
Figures E-G). Seasonal (summer and winter) sculptured egg types
are seen in some species. The time from egg deposition to hatching
is variable between species and due to environmental factors, but
usually ranges from 5 to 40 days. After embryonic development,
the young tardigrade hatches by using stylets, hind legs, or hydrostatic
pressure. Immature eutardigrades appear similar to adults,
whereas immature heterotardigrades are noticeably different. Both
take two to four instars (molts) to become sexually adult.