A monograph of the British fossil corals - kreidefossilien.de

iv
BRITISH FOSSIL CORALS.
In cases of this kind the distinction between the Polypi and the Bryozoa is always
rendered easy by the most superficial examination of the soft parts of the animal ; but it is
sometimes a matter of great difficulty for the palaeontologist, who is
necessarily deprived of
all such resources, and can only be guided by the peculiarities observable in the ossified
tissues.
In general, the distinction between Corals and Spongidae is also very easy, for the
lamellar structure, so prevalent among the former, is never met with in the latter ; but in
some Polypidoms (certain Milleporidse for example), the vertical plates disappear, and the
mural tissue becomes extremely porous, irregulai', and abundant, so as to resemble much
the reticulated mass formed by the stony skeleton of some Spongidae, where the oscula
and aquiferous canals are on the contrary more regular than usual. In cases of this kind
it may be necessary to seek for distinctive characters in the internal structure of the
Zoophyte ; and, independently of the benefit to be obtained by the microscopical investigation
of the tissue itself, it will sometimes be found useful to examine the form of the
tubular cavities which pervade the mass, and correspond either to the visceral chambers of
the Polypi, or to the great aquiferous ducts of the Spongida; ; for in the first instance
they are always simple, Avhereas in the latter they are more or less ramified.
§IL
The external forms of Corals vary considerably, but are in general more dependent on
the mode of aggregation of the different individuals produced by a common parent than
on the mode of organization pecuhar to
the animals to which these tegumentary skeletons
belong. Characters derived from these forms can therefore be but of little avail for the
natm-al arrangement of Polypi ; and the classification of these Zoophytes, like that of the
higher animals, must be founded on the principal modifications observable in their structm-e.
It would lead us too far from the special object of this Monograph, if we were to
enter on the investigation of the anatomical facts which alone can furnish satisfactory
elements for such a classification ; but in order to facilitate the study of the Corals about
to be described, it may be useful for us to revert to a few of the leading points in the
structure of Polyi^i, and to define some of the expressions which we shall often have to
employ.'
The ScLERENCHYMA, or hardened tissue of Polypi, by which Corals are formed, is
always a portion of the tegumentary system of these Zoophytes, but, as we have abeady
stated, it may be produced in two very different ways. In some cases it is the result of
a sort of ossification of the chorion or principal tunic of the Polypi ; in others it grows on
'
For more ample details on this subject we must refer to our " Memoir on the Structure and Development
of Corals," published in the Annales des Sciences Naturelles, 3"° serie, t. ix.