The Polyzoa or Moss Animalcules

Abstract

There are many common marine Polyzoa, some of which are-known as ‘sea-mats ’; the fresh-water forms are less prominent, but they are not really uncommon. The fresh-water species are all colonial, and perhaps several hundred of the individual ‘polyps’ are found aggregated together. The individuals look superficially like Hydras, but further investigation shows that they are very different and much more complex (Fig. 190A). The members of a colony are connected together and protected by a strong sheath secreted by the animals. The most prominent feature of the individual is the lophophore (Fig. 190B), as the horseshoe-shaped crown of tentacles is called. The lophophore is retractable, and if the animal is disturbed it is withdrawn into the protective covering and the polyzooan looks quite shapeless. In a few minutes the animal will probably expand again. The tentacles comprising the lophophore are ciliated, and the cilia make currents which collect the small organisms used as food, and transport them to the mouth. Polyzoa possess a proper gut, which terminates in an anus; ovaries and testes (they are hermaphrodites); and a rudimentary nervous system. The fresh-water polyzoa reproduce asexually by budding — a colony starts with one individual and ‘grows’ in this way.