[21] _Polyzoa_ means "many animals," in allusion to their habit of living in association. _Bryozoa_, "moss-animals," from some forming cells having that appearance. [Illustration: Plumatella repens. Single Polypide enlarged] Polyzoa were formerly associated with the polyps, to which they bear a strong superficial resemblance; but they are of a much higher degree of organization, as will be seen by comparing what has been said in a former chapter on the _Hydra_, with the description which we now proceed to abridge from Dr. Allman's splendid monograph on the fresh-water kinds. In order to get a general conception of a Polyzoon, the Professor tells us to imagine an alimentary canal, consisting of oesophagus, stomach, and intestine, to be furnished at its origin with long ciliated tentacles, and to have a single nervous ganglion on one side of the oesophagus. We must then conceive the intestine bent back till its anal orifice comes near the mouth; and this curved digestive tube to be suspended in a bag containing fluid, and having two openings, one for the mouth and the other for the vent. A system of muscles enables the alimentary tube to be retracted or protruded, the former process pulling the bag in, and the latter letting it out. The mouth of the bag is, so to speak, tied round the creature's neck just below the tentacles, which are the only portions of it that are left free. The investing sack has in nearly every case the power of secreting an external sheath, more or less solid, and which branches forming numerous cells, in which the members of the family live in a socialistic community, having, as it were, two lives, one individual, and the other shared in common with the rest. The whole group of tubes and cells, whatever may be the form in which they are aggregated, is called the _Polypary_, or, as Dr. Allman prefers, the _Coenoecium_ (common house); the creature he names a _Polypide_[22] (polyp-like); and the disk which bears the tentacles _Lophophore_ (crest-bearer). There are some more hard words to be learnt before the student can enjoy himself scientifically among the Polyzoa, and we shall be compelled to employ some of them before we have done; but will now endeavour to describe what was presented to our view by the specimen obtained from the Hampstead Pond. [22] _Polyzoon_ is preferable, as avoiding confusion with _polypite_, used for another class of object.