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 Spongidse 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, irregular, and abundant, so as to resemble much 

 the reticulated mass formed by the stony skeleton of some Spongidse, 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 investi- 

 gation 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 Spongidas ; for in the first instance 

 they are always simple, whereas in the latter they are more or less ramified. 



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 peculiar to the animals to which these tegumentary skeletons 

 belong. Characters derived from these forms can therefore be but of little avail for the 

 natural 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 struc- 

 ture. 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 Polypi, and to define some of the expressions which we shall often have to 

 employ. 1 



The Sclerenchtma, 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 already 

 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 



1 For more ample details on this subject we must refer to our " Memoir on the Structure and Develop- 

 ment of Corals," published in the Annales des Sciences Naturelles, 3 me serie, t. ix. 



