∞ generated and posted on 2023.05.22 ∞

Triploblastic animals that lack body cavities.

Acoelomates need to be distinguished not just from coelomates but also from those animals that lack mesodermal tissue as well, the latter described as diploblastic rather than triploblastic.

That is, acoelomates can be viewed as members of kingdom Animalia that lack coeloms despite possessing mesodermal tissue. As such, acoelomates include particularly the flatworms, that is, members of phylum Platyhelminthes. See also phylum Nemertea.

As noted, diploblastic animals can be considered to be neither acoelomates nor coelomates since they also lack mesodermal tissue. If one looks hard enough, however, one can find members of phylum Cnidaria as well as phylum Ctenophora described also among the acoelomates.

The parazoans (sponges), however, though they also lacking coeloms certainly cannot be described as acoelomates since they lack true tissues altogether and therefore cannot possess cavities within tissues.

Contrast acoelomates also with pseudocoelomates.