Parazoa

(invertebrate zoology) A name proposed for a subkingdom of animals which includes the sponges (Porifera).

A name proposed for a subkingdom of animals which includes the sponges. Erection of a separate subkingdom for the sponges implies that they originated from protozoan ancestors independently of all other Metazoa. This theory is supported by the uniqueness of the sponge body plan and by peculiarities of fertilization and development. Much importance is given to the fact that during the development of sponges with parenchymella larvae, the flagellated external cells of the larva take up an internal position as choanocytes after metamorphosis, whereas the epidermal and mesenchymal cells arise from what was an internal mass of cells in the larva. These facts suggest that either the germ layers of sponges are reversed in comparison with those of other Metazoa or the choanocytes cannot be homologized with the endoderm of other animals. Either interpretation supports the wide separation of sponges from all other Metazoa to form the subkingdom Parazoa or Enantiozoa. On the other hand, there are cogent arguments in favor of the basic similarity of the development of sponges and other Metazoa. See also Calcarea; Demospongiae; Metazoa; Porifera.