14 Dr Harmer on the Structure and Classification 
The operculum is merely in contact with the calcareous wall, 
and retains its original relations in being continuous with the 
membranous floor of the compensation-sac. Numerous muscles 
pass from the vertical calcareous walls of the zooecium to the 
floor of the sac. It may safely be concluded that the contraction 
of these muscles will dilate the sac, thereby introducing water 
into it from the outside and exercising a pressure on the fluid of 
the body-cavity, resulting in the protrusion of the polypide. The 
muscles which produce this effect are thus, in their function as in 
their morphological nature, to be regarded as parietal muscles ; 
and their relation to the compensation-sac is an important link in 
the chain of evidence tending to show that the calcareous “ front 
wall” is derivable from an arrangement similar to that of 
Cribrilinidae. The front wall may be developed in two very 
different ways : — 
(a) In Umbonula verrucosa , it grows over the aperture at a 
higher level, as a continuous overarching lamina, which commences 
proximally and laterally ; the space which it covers always remain- 
ing widely open to the exterior. A Membranipora- like opercular 
wall is present in the young zooecium, and the parietal muscles 
develop in situ. This arrangement is in no way modified during 
the later development, and the operculum still retains its primitive 
condition of having no basal sclerite. So far as I can judge from 
dry material, calcification proceeds in the same manner in certain 
species of Porella, Mucronella and Escharoides ; which are accord- 
ingly to be regarded as related to Umbonula. 
(b) In Lepralia pallasiana , Schizoporella linearis , Euthyris 
obtecta, Catenicella cornuta, and others, a different mode of 
development is followed. The front wall appears at first sight to 
result from the direct calcification of a Membranipoi'a - like 
membranous wall which is usually distinctly visible in the young 
zooecium. The compensation-sac is at first not present, but 
develops from a mass of cells beneath the proximal edge of the 
operculum. These cells soon arrange themselves round a cavity, 
which appears semicircular when seen from above, the diameter 
of the semicircle coinciding with the proximal edge of the oper- 
culum, and the arc curving on the proximal side of the base. 
Numerous muscle-fibres radiate from the walls of this sac to the 
more proximal parts of the lateral walls of the zooecium. The 
sac rapidly grows in a proximal direction until it underlies the 
whole or the greater part of the calcareous front wall. The 
muscles which at first radiated from it are now arranged as two 
lateral series of parietal muscles. The identity of arrangement 
of these muscles and those of other Cheilostomes suggests that 
the phylogenetic origin of the calcareous wall has here been 
modified in ontogeny, in such a way that the development of the 
