The lowest in structure among the Polyps are not Corals, but the single, soft-bodied Actiniae.  They have no solid parts, and are independent in their mode of existence, never forming communities, like the higher members of the class.  It might at first seem strange that independence, considered a sign of superiority in the higher animals, should here be looked upon as a mark of inferiority.  But independence may mean either simple isolation, or independence of action; and the life of a single Polyp is no more independent in the sense of action than that of a community of Polyps.  It is simply not connected with or related to the life of any others.  The mode of development of these animals tells us something of the relative inferiority and superiority of the single ones and of those that grow in communities.  When the little Polyp Coral, the Astraean or Madrepore, for instance, is born from the egg, it is as free as the Actinia, which remains free all its life.  It is only at a later period, as its development goes on, that it becomes solidly attached to the ground, and begins its compound life by putting forth new beings like itself as buds from its side.  Since we cannot suppose that the normal development of any being can have a retrograde action, we are justified in believing that the loss of freedom is in fact a stage of progress in these lower animals, and their more intimate dependence on each other a sign of maturity.

There are, however, structural features by which the relative superiority of these animals may be determined.  In proportion as the number of their parts is limited and permanent, their structure is more complicated; and the indefinite multiplication of identical parts is connected with inferiority of structure.  Now in these lowest Polyps, the Actiniae, the tentacles increase with age indefinitely, never ceasing to grow while life lasts, new chambers being constantly added to correspond with them, till it becomes impossible to count their numbers.  Next to these come the true Fungidae.  They are also single, and though they are stony Corals, they have no share in the formation of Reefs.  In these, also, the tentacles multiply throughout life, though they are usually not so numerous as in the Actiniae.  But a new feature is added to the complication of their structure, as compared with Actiniae, in the transverse beams which connect their vertical partitions, though they do not stretch across the animal so as to form perfect floors, as in some of the higher Polyps.  These transverse beams or floors must not be confounded with the horizontal floors alluded to in a former article as characteristic of the ancient Acalephian Corals, the Rugosa and Tabulata.  For in the latter these floors stretch completely across the body, uninterrupted by vertical partitions, which, if they exist at all, pass only from floor to floor, instead of extending unbroken through the whole height of the body, as in all Polyps.  Where, on the contrary, transverse floors exist in true Polyps, they never cut the vertical partitions in their length, but simply connect their walls, stretching wholly or partially from wall to wall.