The minute worms that live within the tubes in question belong to the genus _Spirorbis_, and are very similar to those of the _Serpulæ_, and their pretty plumed gills may be seen with a lens when a cluster of the tubes is placed in a shallow vessel of sea water. A sharp tap on the table on which the vessel rests will cause the little creatures to suddenly retire into their homes, the entrances to which may then be seen to be closed by an operculum. There is an interesting group of animals known collectively as the _Bryozoa_ or _Polyzoa_, or, popularly, as the Moss Polyps, that are often classed with the worms, though they are not, according to the general idea, wormlike in appearance. They live in pretty colonies, many of which are certainly familiar objects to all who ramble along the sea shore. Some form pretty lacelike patches on the fronds of sea weeds, while others are built up into flat, frond-like, branching objects that are often mistaken for sea weeds by young collectors. Among the latter is the Sea Mat (_Flustra_), that is so commonly washed up on the shore in great abundance. An examination with a lens will show that, in both instances, the mass consists of very many minute cells, with horny or calcareous walls, the mouth of each cell being close by an operculum. On placing the colony in sea water, however, we find that each little cell is the home of a small animal, that protrudes from the cell, exposing a mouth that is surrounded by a crown of tentacles. A moderately high magnifying power will also show that the tentacles are covered with minute vibratile cilia, by means of which currents of water are set in motion towards the mouth to supply the animal with food. Some, too, have a lip by means of which the mouth may be closed. [Illustration: FIG. 124.--_Flustra_ IN ITS CELL, MAGNIFIED] In addition to the colonies just briefly described, there are other moss polyps that build up little, branching, tree-like clusters which closely resemble some of the sea firs, and many of these are to be found in the sheltered crevices of rocks, or attached to the under sides of stones between the tide-marks. While searching the surfaces of rocks and weeds at low tide, one’s fingers will be constantly coming in contact with fixed, soft-bodied animals that suddenly eject a fine stream of water as they are touched. These are the Sea Squirts, sometimes spoken of as the Tunicate Worms. They are semi-transparent creatures of oval or elongated form, and usually of a pale yellow, brown, or pink colour; and derive their popular name from the fact that they are covered externally by a continuous tunic or wall of tough structure.