www.bryozoans.nl - Anatomy

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Definition

[Ryland I] provides the following definition of bryozoans:

"Bryozoa are sedentary, aquatic, colony-forming coelomates. Each colony arises by asexual budding from a primary zooid or ancestrula formed by the methamorphosis of a sexually produced larva, or from a resting bud (statoblast) in the freshwater class Phylactolaemata; the zooids normally remain in communication throughout the colony. Each zooid has a circular or crescentic lophophore bearing a series of slender, ciliated, post-oral tentacles. The anterior part of the body froms an introvert, within which the lophophore and tentacles can be withdrawn. In the Phylactolaemaa the mouth is covered by a projecting flap (epistome). The alimentary canal is deeply looped, so that the anus opens near the mouth, but just outside the lophophore. In most, the nervous system is represented mainly by a small ganglion between the mouth and the anus. Excretory organs are absent. There is no respiratory or circulatory system. Colonies, but not all zooids, are hermaphrodite; simple ductless gonads, derived from the peritoneum, shed gametes in the coelom, which opens to the exterior through coelomopores. Each zooid secretes a rigid or gelatinous wall, so providing support for the colony. The class Gymnolaemata is well characterized by highly developed polymorphism of zooids. Most bryozoa live in the sea; a few inhabit brackish and fresh waters."

The anatomy of a zooid

anatomy The figure below from [Allman] shows the most important elements of the zooid anatomy:
The lophophore with the tentacle crown on top of it is easy to spot. On the sides of the tentacles are 'cilia' or whiphairs that work in concert to create a current towards the mouth.
The lophophore and the tentacle crown can be retracted in to the body cavity by means of the 'retractor muscle'. The other muscles are needed to extend the lophophore and thereby the tentacle crown. The retractor muscle is much larger and stronger than the other muscles. The lophophore and thus the tentacle crown are retracted fast. Extending them takes more time due to smaller muscles working.
Within the lophophore is the mouth. The esophagus, stomach, gut and anus, that is located outside the lophophore, can be clearly seen.
Also the nerve tissue (ganglion) is shown.

The exocyst is the outer layer of the body wall. The endocyst the inner layer.
The exocyst is made of chitin in some species and of a more gelatinous like substance in other species.

The funiculus is a 'ligament', a tendon, that connects the stomach to the bottom of the body wall and that is also associated with the formation of the statoblasts (see below).

The underside of the body cavity is drawn open ended. For most fresh water bryozoan species it is true that an (almost) open interconnection exists between zooids in a colony.

It is important to realize that the body cavity is filled with fluid. This makes retracting the lophophore and the tentacle crown possible. It also facilitates communication between zooids.

What is not shown is that the zooid, or at least a zooid that originated the colony, is attached to the substrate.

Respiration

A zooid does not have special respiratory organs. The zooid is small enough to allow oxygen and carbondioxide exchange with the environment through diffusion.

Statoblast formation

The figure below from [Allman] shows where the statoblasts are formed - in the funniculus.

statoblast formation

Sexual reproduction

The figure below from [Allman] shows where the organs for sexual reproduction are.
The larvae, when mature enough, enter into the outside world through a pore besides the lophophore.

reproductive organs

The anatomy of a colony

The anatomy of a colony The figure below from [Allman] shows a colony of Fredericella sultana.
It is clear that one colony consists of mulltiple zooids that are in open connection with each other. The common tube forks in several places to provide room form more zooids. This is an example of a branching colony. The same principles apply to other species. Sometimes the common tube has disappeared and the zooids are packed densely (e.g. Plumatella fungosa) or the common tube has fused (e.g. Lophopus crystallinus).
At the other extreme Victorella pavida has a shared tube from where the zooids extend. Here the shared tube can clearly be seen and recognized.

P. fungosa P. fungosa top view