BRTOZOA OR POLYZOA. 
117 
may spread in delicate gauze-like sheets over weeds, shells, 
and stones, rise in hard shrub-like tufts, forming hemi- 
spherical masses, or stretch out flexible horny branches. In 
the fossils the soft body of the animal is of course destroyed, 
and there remain only the hardened walls of the little 
chamber in which each zooid lived. Fortunately the shapes 
of these chambers afford characters by which the species 
can be classified. They must, however, be studied under the 
microscope. In the an-angement of the collection, therefore, 
specimens are exhibited to show the general form and habit 
of the colony, and drawings are placed beside them to show 
the minute structure of the chambers. The specimens are in 
most cases near enough to the front of the case to admit of 
the use of a magnifying glass, and thus the main features of 
the chambers can be recognised. 
Setting aside the aberrant Loxosoma and its allies, 
modern Bryozoa are divided into two Sub-Classes : (1) 
PHYLACTOLAEMATA, in which the mouth of the zooid 
has a lip, and the crown of tentacles or lophophore is horse- 
shoe-shaped ; (2) GYMNOLAEMATA, in which there is no 
lip to the mouth, and the tentacles form a comidete circle. 
Since the chambers of the Bhylactolaemata are either soft or 
horny, they are not preserved as fossils, so that we are 
concerned only with the Gymnolaemata. Omitting the 
doubtful Ctenostomata, of which no fossils are certainly 
known, these last are divided, according to the structure of 
the chamber-opening, into four Orders : (1) Trepostomata 
(turned mouths) ; (2) Cryptostomata (hidden mouths) ; (3) 
Cyclostomata (round mouths) ; (4) Cheilostomata (lip 
mouths). 
The Trepostomata and Cryptostomata are all extinct, 
but seem to have arisen from early forms of Cyclostoinata. 
The Trepostomata, which were dominant in early Palaeozoic 
times, generally form massive colonies, composed of the 
chambers drawn out into long tubes and set side by side ; the 
tubes turn xxpwards towards the openings at their ends ; as 
the colony grew each animal moved up in its tube, the lower 
part of which was cut off’ by a platform (diaphragm) like 
the tabula in I'abulate Corals. Examples are Monticulijyora, 
Stenopora, and Callopora (Fig. 62). In the Cryptostomata, 
which became dominant in later Palaeozoic times, the opening 
of the chamber is hidden at the bottom of a tubular shaft 
(vestilnile) ; the chambers grow up into continuous tubes, as 
in the Trepostomes. Examples are IHilodictya, liltahdomeson, 
Gallery 
VIII. 
