The systematic position and relations of these four classes still remains, in spite of much research, a matter of complete uncertainty. They are often grouped together as Molluscoidea. The term is not used here because there does not appear to be any reason to approximate them to the Mollusca. If, however, recent views as to the resemblance between the larval Loxosoma and the Molluscan Trochosphere prove correct, the entoproctous Polyzoa will have to be separated in all probability from the other Polyzoa, as well as from the remaining classes, with the possible exception of Pterobranchia.
The nearly complete atrophy or the total absence of a praeoral region, the presence of a lophophore bearing ciliated tentacles, of a lip or epi-stome overhanging the mouth, and of a fixed mode of life are characteristic of this assemblage of forms. The Brachiopoda and Vermiformia are social, the Polyzoa and Pterobranchia form colonies by gemmation. The two first possess a coelome, which is an enterocoele, and at least one pair of nephridia, the Pterobranchia and ectoproctous Polyzoa a coelome, the entoproctous nephridia but no coelome. As to the lophophore it may be remarked that in Brachiopoda, Vermiformia, and apparently in ectoproctous Polyzoa, it is postoral, in entoproctous Polyzoa praeoral, in the Ptero-branchia doubtful in position on account of the development being as yet completely unknown. The epistome of the entoproctous Polyzoa appears to be homologous' with the foot of the Molluscan Trochosphere, and in Pterobranchia it is not only an organ of locomotion but also secretes, at least in Rhabdoplenra, the tube in which the animal lives. In Brachiopoda and Vermiformia, certainly in the last, it is a remnant of an atrophied praeoral lobe.
The position of the mouth within the lophophoral area, as in Brachiopoda and Vermiformia, would tend to the supposition that the epistome of the phylactolaematous Polyzoa is also a remnant of the same region. It is however often regarded as homologous with the foot of Pterobranchia. The peduncle of the Vermiformia is a ventral growth; there is no reason to suppose that the corresponding structure in Brachiopoda is anything of the kind; nor are the divisions of the coelome homologous in the two classes as has been supposed; nor again is the anus of Brachiopoda to be considered as dorsal in position as it is in Vermiformia.
There is some slight evidence in favour of considering Brachiopoda as primitively segmented animals, not on account of the segmentation of the larva which does not affect the mesoderm, but owing to the presence of two pairs of nephridia in the genus Rhynchonella. It is not a necessary-inference from the fact, inasmuch as there may be more than one pair of nephridia to a somite in some Chaetopoda; but the difference in position of the two pairs of organs rather lends weight to the view. The presence of a pair of otocysts in the larvae of the hingeless genera, of setae in the adult, but more especially of provisional setae in most of the larvae, are points which should also be noted in the anatomy of Brachiopoda. Setae are not necessarily characteristic of the Vermian Chaetopoda; e. g. they occur in Chiton among Mollusca, but provisional larval setae are not met with elsewhere. The presence of the structures named tends rather to the supposition, when coupled with other facts, that Brachiopoda are a group of simplified or degenerated organisms.
It is better under these circumstances of doubt to retain the four classes in question apart by themselves, but it must be carefully borne in mind at the same time that there is no evidence worth much to connect them one with another.