In Heterotardigrada, gonoducts open to the outside through a preanal gonopore. Males possess two vas deferentia, a swollen portion serving as a seminal vesicle. Females possess a single oviduct which passes to the right of the intestine. In accordance with the small size of tardigrades, there is no respiratory or circulatory system. For some species of tardigrades, no males have been found.
Parthenogenesis occurs in two fashions.
In diploid tardigrades, meiosis distributes chromo- somes hetween the oocyte and the polar body. The chromosomes are duplicated, then a second mitotic divi- sion occurs, yielding a dipoid egg nucleus and polar body. In triploid tardigrades, oocytes are formed by mitotic divisions, yielding triploid eggs. Fertilization is external, hut protected. As a female is molting, males may introduce spermatozoa into the gonopore or cloaca.