1(38 
THE VOYAGE OF H.M.S. CHALLENGER. 
The zoarium appears to be dimorphous, or rather polymorphous, as it sometimes 
encrusts foreign bodies, in a single unattached layer. But even in its independent 
erect form, or more complete condition, as it may be termed, it is extremely variable 
in habit. Sometimes it presents long cylindrical distant branches, and sometimes 
assumes the form of stunted tufts or short branches, or even little more than 
irregular nodulated masses. 
On a transverse section the interior of a branch exhibits no central axis nor inter- 
laminar expansion. The interzooecial walls are extremely delicate and closely perforated 
in all parts. 
In the dried condition the cavity of each zooecium is seen to be divided into two 
compartments, an anterior and a posterior, by a vertical diaphragm formed of an extremely 
delicate membrane. The hinder compartment, which in the dry state is very much the 
larger, lodges the polypide and communicates directly with the oral orifice, whilst the 
anterior, which probably exists as a chamber only in the dry state, being merely the 
space between the vertical diaphragm and the anterior calcareous wall, in the ordinary 
zooecia does not appear to contain anything, but in others it is into this space that the 
subjacent ooecium intrudes, and may often be seen in the form of a spherical vesicle 
containing a vitelline mass or embryo. As a similar arrangement exists in Siphonicytara 
it is not improbable that this division of the zooecial cavity into two compartments by 
a flexible membranous diaphragm will be found pretty generally in all zooecia of which 
the wall is wholly rigid, and that it is intended for the purpose of allowing the compres- 
sion of the perigastric cavity necessary to effect the protrusion of the polypide, which 
would otherwise seem to be scarcely possible in a perfectly unyielding box. 
11. Myriozoum, Donati. 
Myriozoum , Donati, 1750, d’Orb., Smitt (pars). 
Millepora (pars), Pallas, 1766, Solander, Strom, Fabricius. 
Myriapora, Blainv., 1834. 
1 Foricala (pars), d’Orb. 
Cellepora (pars), Leieschara , Sars. 
Gemellipora (sp.), Smitt. 
Character . — Zoarium erect, branched, continuous ; branches cylindrical, obtuse ; or 
oviform. Surface punctured or reticulate. Avicularia, when present, immersed and 
usually placed near the orifice, either above, below, or on one or both sides. Orifice 
notched or sinuate, or canaliculate below. 
The transition from the typical form of Myriozoum as exhibited in the well known 
Myriozoum truncatum to other cylindrical escliarine growths, is so gradual that it is in 
