For educational purposes only;
Information on the basics of Invertebrate Zoology
Kingdom: Animalia, Phylum: Bryozoa
CLICK on underlined Plates to view and included illustration to enlarge:
Members of the phylum Bryozoa have been referred to as "Moss Animals" and "Ciliated Crowns." Two groups have been separated: The Ectoprocta are those with the anus located on the outside of the lophophore and the Endoprocta where the anus occurs inside of the lophophore. The Endoprocta are no longer generally recognized as a group, however.
These organisms form low, crusting mats over leaves, sticks, etc. in the water. They occur in marine environments and in slow, sluggish freshwater streams. Although individually microscopic, they form macroscopic colonies.
There are two body forms: Zooida and Zooecium. Individuals of the Zooida resemble polyps of Ctenophora or Cnidaria superficially. They secrete a case around themselves, which is called the zooecium. The zooecium is continuous in the colony of some species and a large size may be assumed at times. This is composed of chitin-like material, gelatin or calcium carbonate, all of which are proteinaceous in the marine environment. In fresh water forms this is always gelatinous.
Characteristics of the Phylum
Regarding Body form, the whole upper portion of the body is capable of being withdrawn into the zooecium. Then the exposed portion is called the introvert, which is covered by an extremely thin case consisting of the same material as the zooecium. Tentacles form around the mouth in the form of either a complete ring or horseshoe shape (= lophophore).
The Body Wall consists of a simple epidermis, which secretes the zooecium. Thin layers of circular and longitudinal muscles are well developed on the introvert only. The peritoneum lines all the interior portions of the coelom.
A True Coelom is present, which is the first case of this being found in the present organization of Invertebrates. It is completely lined with peritoneum and is formed of mesoderm. The peritoneum also covers the digestive organs and all inner portions.
Food and Digestion.-- Microscopic animals are gathered by cilia on the tentacles. These cilia not only force food down the gullet but also stir it up in the gut. The mouth of the digestive tract and the anus are positioned close to each other. There is a looped, U-shaped digestive tract. The entire gut is ciliated throughout.
Circulation.-- There is no circulatory system present, and food is move by fluid in the coelom.
Respiration.-- There are no special respiratory organs. Any exposed surface serves here, which is usually afforded by the tentacles.
Excretion.-- There are no excretory organs. Excretion is accomplished by diffusion.
Support & Protection.-- The zooecium provides both support and protection. Some protection may also be afforded by the avicularium (see below).
Movement.-- There is no locomotion. Well-developed retractor muscles are present, which pull the introvert down into the zooecium. Muscle layers occur over the entire body.
Avicularium.-- Also known as the "little bird," this structure is found only in marine forms. It is a bird-like beak that serves to ward off other animals or plants, which would settle-down and grown on the organism. This serves to keep the colony clean. It is believed to be a highly modified zooid.
Nervous System.-- The simple nervous system consists of a single nerve ganglion that is situated between the mouth and the anus. There are no visible sense organs, but the exposed body surface is highly sensitive to stimuli.
Reproduction.-- Bryozoa are hermaphroditic. The Feniculus, which holds the stomach in place, also bears the testes. The ovaries occur on the body wall. Self-fertilization may occur but is not substantiated.
Life Cycle.-- In the Sexual Cycle, the fertilized egg develops into a ciliated larva. In the genus Bugula, the larva develops in an ooecium at the base of the zooecium. In Plumatella it develops inside of the zooecium. The larva breaks out through a temporary hole in the body wall. It then swims about for a few days and settles down. Later it gives rise to the whole new colony by asexual reproduction.
In the Asexual Cycle, freshwater Bryozoans (e.g. Plumatella), bud to form statoblasts. These are produced on the feniculus similar to the gemmules of the Porifera. Statoblasts are covered with spines. They remain in the interior of the body and carry the organism over dry seasons when the parent disintegrates. Asexual reproduction is much more common than sexual reproduction in freshwater forms.
Importance.-- Bryozoa are of no economic importance but have great academic interest. The genera Plumatella, Cristatella and Pectinatella are widely studied.
Comparison of Marine with Freshwater Varieties.--
The freshwater genus Plumatella is a colonial organism, which has a common and continuous cavity throughout the entire colony. Three is a gelatinous zooecium and the lophophore is horseshoe-shaped. Larvae develop at the base of the zooecium on the interior. There is no avicularium but statoblasts are present.
The marine genus Bugula is colonial. Individual bush like cases are distinct and not in communication. There is a chitinous zooecium and the lophophore is a complete ring. Larvae develop in the ooecium. There is an avicularium but no statoblasts are present.
Please see following plates for Example Structures of the Bryozoa:
Plate 34 = Phylum: Bryozoa: Plumatella sp. & Cristatella mucedo
Plate 35 = Phylum: Bryozoa -- Example Structures: Pectinatella sp. & Bugula sp.