9.1 Introduction

Bryozoa, also known as the Ectoprocta, is the only phylum in which all known species are colonial. A colony consists of very small individuals called zooids that are physically connected and produced asexually. Some or all zooids in a colony-these are often called feeding colonies- are organized as functional and morphological units capable of feeding and carrying out all other vital processes.

Colonies may be encrusting to frondose (shrub-like), and even free-living. All are aquatic; the majority are marine. The fossil record of the Bryozoa begins in the Early Ordovician, and they are common today.

Zooids within a colony can differ distinctly in morphology and function. Some, if not all, are feeding colonies. However, many colonies possess polymorphic zooids that differ morphologically and perform specialized functions, including reproduction, colony support, cleaning and defense.

Feeding zooids have a fluid-filled body cavity (coelom), a protrusible tentacle-bearing feeding organ called the lophophore, a U-shaped digestive tract, muscles, a nervous system, and tissues attaching the digestive tract to the body-wall. Eggs and sperm producing organs are present in some feeding and non-feeding zooids in all colonies.

The lophophore consists of a ring of hollow tentacles and a supporting tentacle sheath. The sheath encloses the tentacles when they are retracted, and turns inside out for support when they protrude for feeding. The tentacles are covered with cilia that produce currents directed toward the mouth centered at the base of the tentacular ring. The mouth opens into the U-shaped digestive tract which ends at an anus on the side of the protruded tentacle sheath below the ring. Protrusion of the tentacles involves exerting hydrostatic pressure on the fluid of the body cavity. This pressure is produced in various ways, usually by muscles modifying the shape of the body cavity. Other muscles contract to bring about retraction of the tentacles.

In many modern bryozoans, the polymorphic zooids include kenozooids, which consist of body walls enclosing body cavities but with no lophophore. Kenozooids commonly serve as small space-fillers or as long tubular anchoring rootlets. Another polymorph is the avicularium, which resembles to some extent a bird's head. Muscles open and close a beak-like mandible that snaps to clean and defend the colony. Brood chambers are present in many bryozoans. Fertile eggs are housed in these chambers until a floating larva escapes. The larvae of marine bryozoa are ciliated and planktonic. After settling, a larva undergoes extensive reorganization of tissues to produce the first zooid, which is called the ancestrula of the colony. This first zooid then buds asexually to produce the succeeding individuals. In most freshwater bryozoa, few colonies originate sexually. Most colonies form from asexually-produced resistant bodies called statoblasts which can survive unfavourable conditions, such as freezing and dessication. In many marine and freshwater byrozoans, colonies may arise from fragments of pre-existing colonies.

9.2 Classification and Taxonomy

Modern classifications of the Bryozoa recognize three classes within the phylum. The marine species are grouped into the classes Stenolaemata and Gymnolaemata. They are distinguished primarily by features of their soft-anatomy. The third class, Phylactolaemata, includes only freshwater species. In these the mouth is covered by a fleshy hood and the tentacles may be arranged in a bilobed or circulax ring about it. No hard parts are mineralized by species in this class, but they have left a fossil record of imprints and tubes in deposits of Tertiary age.

9.2.1 Class Stenolaemata

Because bryozoan classification into classes centers primarily on soft-anatomy, only selected orders will be discussed. The following four orders are assigned to Class Stenolaemata.

Order Trepostomata

• These are the so-called "stony" bryozoans of the Early Paleozoic. Most formed massive to dendroid calcareous colonies. The feeding zooids were housed in long, circular to polygonal tubes in which transverse partitions, called diaphragms, are present. There are relatively few diaphragms in the inner zones of the tubes, but they are numerous and closely-spaced in the outer parts. Trepostomes appeared late in the Early Ordovician and diversified rapidly to a maximum in the Middle and Late Ordovician. They were the most abundant fossil group in some Ordovician beds. Trepostome diversity was reduced greatly in the Silurian, made a partial recovery in the Devonian, and then declined markedly to two or three genera in the Triassic, when the trepostomes became extinct.

• These form delicate, less highly-calcified colonies than the trepostomes. They are characterized by short zooidal tubes in which the aperture is hidden or obscured in a depression called a vestibule. Diaphragms are commonly absent. A hemiseptum, a short plate that extends part way across the aperture at the top of the zooidal tube, may be present. These byrozoans range from the Ordovician into the Permian.

• The zooidal tubes in this order are similar to those in the Cryptostomata, although the chambers for feeding zooids are even smaller and are circular in cross-section. Colony form is sheet-like or fanlike in many species. Fenestrates appeared in the Early Ordovician, increased slowly in abundance into the Late Paleozoic, and became extinct during the Triassic. Their maximum abundance was reached in the Lower Carboniferous.

• Cyclstomes have plain rounded apertures, thin zooidal walls, and usually an absence of diaphragms.

  Colony form is variable. Most tube walls have very small communication pores. Cyclostomes first appeared in the Ordovician and formed small, insignificant colonies throughout the Paleozoic. During the Cretaceous, the Cyclostomata underwent an evolutionary explosion (adaptive radiation?). The number of genera recognized quadrupled and the Order reached maximum diversity. Cyclostomes declined at the end of the Cretaceous, with the number of genera dropping from about 175 to 50, a number that has remained more or less constant to the present. This is the only order of stenolaematans known from Jurassic and younger rocks.

The gymnolaematans are the most morphologically varied of the three bryozoan classes. The simplest have no skeletons, no polymorphs, and no change in zooidal form during colony development. They occur as flat encrusting colonies dependent upon the substrate. The most complex gymnolaematans have elaborately calcified skeletons (many zooidal tubes are formed from interlayering of calcite and aragonite), at least two kind of polymorphic zooids, and they display considerable variation of zooidal form during colony development. The greater part of the class' diversity occurs during the Cretaceous and Cenozoic. The Class Gymnolaemata includes two orders. Order Ctenostomata

• Ctenostomes generally lack skeletons and grow immersed in calcareous substrates such as shells, or as encrustations. One genus is free-living. Colonies immersed in shells commonly consist of widely-spaced feeding zooids that are connected by tubular stems, termed stolons. In living species, the stolons are kenozooids. The order ranges from the Upper Ordovician to the present. There are about 50 genera, most of which are known only from living specimens.

• Most cheilostomes have box-like colonies and an aperture closed by an operculum (lid). Special muscles may attach body tissues to the walls. Walls may be organic material alone or with calcium carbonate. Brood chambers and polymorphic zooids are commonly present. The growth habit includes encrusting to erect, shrub-like forms. Cheilostomes appear first in Late Jurassic rocks with only a few taxa, but the group radiated markedly in the mid-Cretaceous. In some marine sediments, cheilostome fossils may be the most abundant remains of megascopic marine invertebrates and a major rock-forming component.

  The geologically oldest bryozoans are well-calcified stenolaematans of Early Ordovician age. There are few species with highly variable morphology. By Late Ordivician time, however, numerous morphologically distinct taxa had appeared. Trepostomes were the prominent bryozoans of the Middle and Late Ordovician. Fenestrates were common in the Late Paleozoic. Cheilostomes radiated rapidly after appearing, possibly from a ctenostome ancestor, in the late Jurassic.