BRYOZOA OR POLYZOA. 



117 



and stones, rise in hard shrub-like tufts, forming hemi- Gallery- 

 spherical masses, or stretch out flexible horny branches. In VIII. 

 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 arrangement 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) 

 PHYLACTOIjAEMATA, in which the mouth of the zooid 

 has a lip, and the crown of tentacles or lophophore is horse- 

 shoe-shaped ; (2) GYMIfOLAEMATA, in which there is no 

 lip to the mouth, and the tentacles form a complete circle. 

 Since the chambers of the Phylactolaemata 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 some of each Order seem to have given rise to the 

 Cyclostomata and Cheilostomata respectively. The Tre- 

 postomata^ 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 upwards 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 Tabulate Corals. Examples are Monticulipora, 

 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 

 (vestibule) ; the chambers grow up into continuous tubes, as 

 in the Trepostomes. Examples are Ptilodictyct, Rhabdomeson, 



