In the vast majority of the polyzoa, marine as well as freshwater, movement is practically confined to the polypide, the external walls of the zooecium being rigid, the zooecia being closely linked together and the whole zoarium permanently fixed to some extraneous object. In a few freshwater species belonging to the genera _Cristatella_, _Lophopus_, _Lophopodella_ and _Pectinatella_, the whole zoarium has the power of progression. This power is best developed in _Cristatella_, which glides along with considerable rapidity on a highly specialized "sole" provided with abundant mucus and representing all that remains of the ectocyst. It is by no means clear how the zoaria of the other genera move from one place to another, for the base is not modified, so far as can be seen, for the purpose, and the motion is extremely slow. It is probable, however, that progression is effected by alternate expansions and contractions of the base, and in _Lophopodella_ (fig. 32), which moves rather less slowly than its allies, the anterior part of the base is raised at times from the surface along which it is moving. The whole zoarium can be released in this way and occasionally drops through the water, and is perhaps carried by currents from one place to another in so doing. So far as the polypides are concerned, the most important movements are those which enable the lophophore and the adjacent parts to be extruded from and withdrawn into the zooecium. The latter movement is executed by means of the retractor muscles, which by contracting drag the extruded parts back towards the posterior end of the endocyst, but it is not by any means certain how the extrusion of the lophophore is brought about. In most ctenostomes the action of the parietal muscles doubtless assists in squeezing it out when the retractor and parieto-vaginal muscles relax, but Oka states that protrusion can be effected in the phylactolæmata even after the zooecium has been cut open. Possibly some hydrostatic action takes place, however, and allowance must always be made for the natural resilience of the inverted portion of the ectocyst. Even when the polypide is retracted, muscular action does not cease, for frequent movements, in some cases apparently rhythmical, of the alimentary canal may be observed, and in _Hislopia_ contraction of the gizzard takes place at irregular intervals. When the lophophore is expanded, the tentacles in favourable circumstances remain almost still, except for the movements of their cilia; but if a particle of matter too large for the mouth to swallow or otherwise unsuitable is brought by the currents of the cilia towards it, individual tentacles can be bent down to wave it away and similar movements are often observed without apparent cause.