[230] The basis, framework, or endo-skeleton, of these groups of animalcules is termed the _polyparium_, or _polypidom_ (polype-habitation); those of a stony hardness are familiarly known as corals; these names, therefore, refer to the durable substance, and not to the animals themselves; but in familiar writing, the term Coral is often used to designate the entire living mass. The Red-Coral forms a distinct genus called _Corallium_. In fossils, the _polyparium_ alone remains, except in very rare instances. It may here be necessary to notice a prevailing error, regarding the mode in which the substance called coral is produced. It is very generally supposed that Corals, particularly those bearing stars and cells, have been constructed by animalcules, in the same manner as is the honey-comb, by the Bee; and the expressions often employed by naturalists, of "the coral animalcules building up their rocky habitations," and "constructing their cells," have contributed to foster this error. But the processes are in no respect similar: the insect, under the guidance of an unerring instinct, resulting from its peculiar organization, _constructs_ its cells; but the polype is incapable of forming, or even modifying, its support or cell in the slightest degree. The polypidom is _secreted_ by the animal tissues, in the same manner as are the bones in the vertebrated animals, without the individual being conscious of the process. If a piece of white coral be immersed in dilute hydrochloric acid, the calcareous part is dissolved, and the secreting membrane, in the form of a flocculent substance, is seen attached to the undissolved part; even in some coralline marbles of incalculable antiquity, the animal membrane may, in this manner, be detected.[231] [231] See Pict. Atlas, pl. xxxiv. _fig._ 2. From the delicate and perishable nature of many of the gelatinous zoophytes, numerous tribes may have inhabited the seas, which deposited the fossiliferous strata, and yet no indications of their existence remain; while, of others, but obscure traces of their structure are likely to be detected. The Polypifera are separated into two natural groups or classes; viz. the Anthozoa (flower-animals), and the Bryozoa (moss-animals), or Polyzoa. The Anthozoa are polypes of the most simple type of structure. The body consists of a symmetrical gelatinous sac, capable of contraction and expansion, with one aperture or mouth, which is encircled by tentacula. The Hydra, or fresh-water polype (_Wond._ p. 600), is a familiar example of a single, locomotive, anthozoan animal. In the compound or aggregated forms, the body is either inclosed in a horny sheath (ex. _Sertularia_, _Wond._ p. 615), or is supported by a lamellated calcareous endo-skeleton (ex. _Fungia_, _Wond._ p. 623, pl. vi. _fig._ 15), or the soft parts invest a stony axis (ex. _Madrepora_, _Wond._ p. 620), or a horny flexible framework (ex. _Gorgonia_, _Wond._ p. 616).