Bryozoan morphology

The hard parts of an individual zooid are called the zooecium (plural zooecia), and the skeletal colony the zoarium (Fig. 5.2). The zooecia of stenolaemates are tube shaped and are often studied in thin section. In branching colonies, the mature parts of the zooecia usually grow at a high angle to the axis of colony growth. Zooecia may change in shape as they grow. They share common skeletal walls with adjacent zooids. In cross-section these tubes can be identified along with the shared hard tissue between.

In gymnolaemates, the box-like zooecia usually develop nearly parallel to the growth axis of the colony. The upper surface of the zooecium may be protected by spines or a calcite shield to deter predation. In addition, there is a lid or opercu-lum that fits over the aperture from which the zooids emerge. Different types of aperture or zooecia characterize different specialized zooids. More zooecia are added as the colony clones new individuals, and each one quickly reaches a finite size where growth ceases. As a result, the colony builds outwards in a modular fashion, adding building blocks of a standard size.

The shape of the colony, or zoarium, depends on the species and on the environment in which an ancestrula settles. Sticklike, fan-shaped, disk-shaped, and encrusting colony forms are common although they may be difficult to relate straightforwardly to taxonomy or ecology.

Zooid extended, and filter feeding with a lophophore

Muscular collar —

Zooid retracted

Retractor muscles

Nerve ganglion

Anus

Ovary

Nerve ganglion

Anus

Ovary

Testis

Zooid extended, and filter feeding with a lophophore

Muscular collar —

Zooid retracted

Retractor muscles

Stolon

Tissue connections between zooids

Zooecium

Testis

Stolon

Tissue connections between zooids

Zooecium

Tangential section

Transverse section

Longitudinal section

Zooid with lophopore extended

Frontal membrane contracts to extend lophopore

Longitudinal section

Zooid retracted

Soft tissue connection

Zooid retracted

Soft tissue connection

Fig. 5.2 Hard- and soft-part morphology of bryozoans: (a) ctenostome gymnolaemate zooids, (b) sections of a stenolaemate zoarium, and (c) gymnolaemate zooids.

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