1888 - 89 .] Mr G. Brook on a New Type of Dimorphism. 79 
colonial forms, however, do occur, hut so far as is known none of 
them are dimorphic. The Madreporaria include both colonial and 
solitary forms. Our knowledge of the structure of the zooids is as 
yet confined to a few species, and so far as I am aware the only 
known case of dimorphism is that described by Fowler in Madre- 
pora Durvillei. In this case the specialisation affects certain of 
the mesenteries in the modified individuals. In the normal zooids 
all the mesenteries are similar in structure. In the modified zooids 
six out of twelve mesenteries (alternate ones) become thickened, 
and contain a ciliated ectodermal canal running through their sub- 
stance, and opening at both ends into the stomodseum. Apparently 
both types are nutritive and both are reproductive, hut Fowler is 
of opinion that the normal zooid is more reproductive, and the 
modified zooid more nutritive. Thus in this, the only case of 
dimorphism previously described amongst the Zoantharia, there is 
no complete specialisation into nutritive and reproductive zooids, and 
it is of interest to note that in another species of Madrepora 
(M. aspera ), examined by Fowler, a similar dimorphism does not 
exist. 
A study of the “ Challenger” collection of Antipatharia has shown 
that some of the genera are dimorphic, whilst others are not. 
Nearly all the species obtained at great depths are dimorphic, hut 
others come under the same category which occur in shallower seas. 
The nature of the dimorphism and the manner in which it has 
probably been produced will be best understood by a study of the 
general morphology of a few typical forms. 
Most Antipathidce have the horny axial skeleton more or less 
branched, and the zooids are usually situated in a single linear 
series on the branches, and are connected together by a coenenchyma 
which contains an axial prolongation of their coelentera, neighbour- 
ing zooids being thus brought into communication with one another. 
A vertical mesogloeal partition is usually present between adjoining 
zooids, but is never so complete as to prevent intercommunication. 
In a typical zooid there are six tentacles and three pairs of well- 
developed mesenteries, and, in addition, there may he two or three 
other pairs, which are always more or less rudimentary. The 
stomodseum is elongated in the sagittal axis, and usually occupies a 
position at right angles to the axis of the branch on which the 
