INTRODUCTION. il 
centre of the ramified mass produced by the multiplication of these plant-like animals. The 
dendroid red Coral of the Mediterranean Sea and the horny skeleton of Gorgonia are thus 
inclosed in the axis of cylindrical branches, formed by the thick coriaceous tegumentary 
tissue belonging to the whole community of aggregate Polypi, and studded, as it were, by 
the radiate protractile heads of the many imdividual Zoophytes thus united. Other 
Corals, appertaiming either to simple or to compound Polypi, are, on the contrary, pro- 
duced by the ossification of this tegumentary tissue itself, and instead of forming a sort of 
stem, constitute a sheath, or an assemblage of calcareous tubes, each of which belong to an 
individual Zoophyte, correspond to the lower part of its digestive cavity, and serve as a 
kind of cell or lodge into which the anterior portion of the animal’s body recedes when in 
a contracted state. 
The basal or stalk-hke Corals are in general well characterised by their dendroid form, 
compact tissue, and concentric layers. At first sight they may bear a slight resemblance 
to certain Bryozoa that have attaimed a very advanced age ;! but even then the remains of 
some non-obliterated cells will always enable an attentive observer to recognise the latter, 
and the absence of all trace of any such cavities can easily be ascertained, by grinding down 
or fracturmg the stem of the above-mentioned Zoophytes. In some few istances these 
basal Polypidoms are more like the reticulated skeleton of certain foliaceous Spongidee ; but 
the concentric lamella of their stem contrasting with the fibrous structure of the tissue of 
the Sponge, will still render them recognisable. 
Dermal Corals are im general characterised by features of a more striking aspect, 
and it is only when these Polypidoms are reduced to their most simple and degraded form, 
that they can be mistaken for the tegumentary skeleton of some of the lowest Bryozoa, or 
the reticulate, stony tissue of some highly-organized Spongide. In all well-developed 
Corals of this kind, the central cavity or visceral chamber is more or less completely 
divided by a certain number of vertical plates, which project from its walls towards its axis, 
and produce that radiate structure which is so remarkable in the Astrean tribe. In most 
Bryozoa the mouth, or cephalic aperture of the tegumentary cell, is provided with a horny 
operculum,” but no such organ ever exists in a true Coral; and, on the other hand, the 
radiate septa which we have just alluded to as bemg conspicuous im most Polypidoms, 
never exist in the cells of Bryozoa. The absence of an operculum, or of vertical septa, 
will not, however, enable the observer to decide whether the coral-like organic remains 
submitted to his mvestigation belong to the one or to the other of the two great zoological 
divisions, for it is a well-known fact that, in many of the inferior forms among recent 
Bryozoa, the tegumentary skeleton is reduced to a simple non-operculated tubular sheath, 
and that in certain Polypi (the Tubipora for example), no longitudimal septa are to be 
found; and the Polypidom is equally reduced to a calcareous tube, tapering and closed 
at its base, open and more or less enlarged at its upper end. 
1 The Millepora truncata of Ellis and Solander, for example. 
* See “ Recherches sur les Eschares,” Annales des Sciences Naturelles, 2”° série, t. vi, pl. i. 
