This division includes only the singular marine genus Rhabdopleura, in which the lophophore is crescentic, and carries a discontinuous series of tentacles; the mouth is lateral rather than terminal; a special shield-like organ is attached to the body of the lophophore, between the mouth and the anus ; the coenoecium is chitinous and tubular, and is supported by a correspondingly divided chitinous rod, attached superiorly to a fleshy contractile cord, which is in turn connected with the body of the polypites; and, lastly, the endocyst and tentacular sheath are wanting.
Of the above divisions of the Polyzoa, the two most important groups are those of the Phylactolaemala and Gymnolaemata. In the former of these are included almost all the fresh-water Polyzoa, and the lophophore is bilateral and horse-shoe-shaped in all except Fredericella. The division of the Gymnolaemata, on the other hand, includes the fresh-water genera Paludicella and Urnatella, and the vast majority of the marine Polyzoa. Of these latter, the sub-order of the Cheilostomata is the most important, as embracing the greater number of the common forms. In these, the opening of the cell is sub-terminal, and is generally closed by a movable lip or shutter. On the other hand, in the sub-order Cydostomata, the cells are tubular, the orifices terminal, of the same diameter as the cell itself, and without any movable apparatus for closure. Lastly, in the singular group of the Ctenostomata (including Vesicularia, Alcyonidium, and Val-keria), the cells arise from a common tube, and their mouths are terminal, and furnished with a setose fringe for their closure.
By many zoologists the Polyzoa are now regarded as being an anomalous class of Worms, closely related to the true Annelides. That there are points of relationship between these apparently diverse groups cannot be doubted; but these do not seem sufficient to outweigh the points of divergence - such, for example, as the absence of segmentation in the former, and the totally different form of the nervous system. The Polyzoa have also striking affinities with the Brachiopoda and Tunicata, and even with some of the Mol-lusca proper; and it has not, therefore, appeared advisable to remove them to the division of the Anarthropoda.
The Polyzoa, like all the Molluscoida, are exclusively aquatic in their habits, but unlike the remaining two classes, they are not exclusively confined to the sea. The marine Polyzoa are of almost universal occurrence in all seas. The fresh-water Polyzoa, however, not only differ materially from their marine brethren in structure, but appear to have a much more limited range, being, as far as is yet known, principally characteristic of the north temperate zone. Britain can claim the great majority of the described species of fresh-water Polyzoa, but this is probably due to the more careful scrutiny to which this country has been subjected. Fresh-water Polyzoa have also been found in the southern hemisphere, in Australia and India.
The Polyzoa have left abundant traces of their past existence in the stratified series, commencing in the Lower Silurian rocks and extending up to the present day. The Oldhamia of the Cambrian rocks of Ireland, and the Graptolites, have been supposed to belong to the Polyzoa ; but the former is very possibly a plant, and the latter should be referred to the Hydrozoa. Of undoubted Polyzoa, the marine orders of the Cheilostomata and Cyclostomata are alone known with certainty to be represented. Several Palaeozoic genera - such as Fenestella (the Lace-coral), Ptilodictya, Ptilopora, etc. - are exclusively confined to this epoch, and do not extend into the Secondary rocks. Amongst the Mesozoic formations, the Chalk is especially rich in Polyzoa, over two hundred species having been already described from this horizon alone. In the Tertiary period, the Coralline Crag (Pliocene) is equally conspicuous for the great number of the members of this class.