1888-89.] Mr G. Brook on a New Type of Dimorphism. 81 
stomodseum frequently corresponds with the transverse instead of 
with the sagittal axis, as is also the case in Amphianthidce amongst 
the Actiniaria. 
In Sehizopathes the three lobes of the zooid in Parantipathes 
become separated from each other by a further depression in the 
peristome, and also by the formation of a mesogloeal septum, which 
projects downwards for some distance from the base of the depres- 
sion towards the skeletal sheath. By these means each lobe of the 
primitively simple zooid becomes separated from its neighbour in 
the same manner as the simple zooids of Antipathella, Paranti- 
pathes, , &c., are separated from each other. The three lobes of the 
zooid in Parantipathes , having become separated from each other in 
Sehizopathes by the formation of vertical mesogloeal partitions, may 
now be considered dimorphic forms. The middle one containing 
the stomodseum, which opens by the mouth at the apex of an elon- 
gated tubular projection, may be termed the gastrozooid, whilst the 
two lateral ones, containing the reproductive organs, may be dis- 
tinguished as gonozooids. Each of the three dimorphic zooids bears 
a pair of tentacles, but the gastrozooid is the only one possessing a 
permanent opening to the exterior. In Sehizopathes the dimorphic 
individuals are arranged in a single linear series along one aspect of 
a branch ; all are in communication with one another through the 
bases of their coelentera, and there is typically no isolation of the 
zooids into triplets, but all are pressed closely together. In specimens 
in which the reproductive elements are well developed, the gono- 
zooids become much distended, and the sequence of the dimorphic 
forms along a branch is then readily recognised. Using the letter 
R to represent the gonozooid and S the gastrozooid, the arrange- 
ment may be indicated in the following manner, the derivatives of 
a primitive zooid being included within brackets : — 
In Bathypathes the differentiation is carried a step further, on 
account of the fact that the individual zooids are separated from 
vol. xvi. 25/3/89 f 
