^T. digitale.^ Size one inch to one and a half inches in height; rose-colored; the bell thin and hard, and conical at the top. The swimming is effected by contractions of the muscular velum (the band around the inner margin of the umbrella) instead of wholly by the bell. The tentacles are long and numerous, and are curled up when moving. The manubrium is long and has four expansions at the mouth. Eight egg-cases [pg129] hang in long pendent sacs from the upper part of the radial canals and reach nearly to the velum. Four garnet-colored eyes in club-shaped processes are prominent on the margin. The animal moves by jerks in straight lines. ORDER =HYDROCORALLINA= CALCAREOUS HYDROIDS [Illustration: /Trachynema digitale./] The genus ^Millepora^ ("thousand pores"), which is the type of this order, is a colony of animals, like other hydroids, which secrete calcareous instead of horny coverings. It differs from true corals in that the members of a colony perform different functions, whereas in true corals each member of a community is a complete individual. It differs also in the arrangement of the stony partitions, which in =Hydrocorallina= are the outside coverings and connecting canals, but in true corals are vertical partitions inside the animal, between the inner and outer sacs, as explained on page 114. GENUS ^Millepora^ ^M. alcicornis^, elk-horn coral. This beautiful coral, which is abundant in Florida and contributes to the building of the reefs, rises in broad expansions, more or less lobed, and suggests by its shape the object for which it is named. The whole mass is porous, being traversed by innumerable canals. Its surface, although smooth compared with that of other corals, is covered with very minute pores, which are of two sizes. The larger ones are the /gastropores/, or stomach-pores, in which the nutritive animal lives; it has a cylindrical body, with four knob-like tentacles and a mouth. Placed more or less irregularly around the gastropores are smaller pores, the /dactylopores/ (finger-pores), from which emerge slender mouthless processes, or /dactylozoöids/, with tentacles and stinging-cells. These seem to be the guard-polyps of the community. The cups occupied by the zoöids are shallow. As one animal dies, another succeeds it and builds a horizontal partition separating the new cup from the old one. Thus the stony mass increases in size by the progress of succeeding generations of zoöids. The living animal occupies only the outer, open space. (Plate XLIV.) [pg130] ORDER =SIPHONOPHORA= FREE-SWIMMING COLONIES This order of hydroids consists of free-floating communities and is one of peculiar interest, since it shows in a clear manner the special function of each individual member of the colony, and illustrates better than the foregoing, perhaps, the curious forms of animal life which this class presents. GENUS ^Nanomia^ [Illustration: Adult /Nanomia cara/.]