Buccal organ

from Wikipedia, the free encyclopedia

The buccal organ is a morphologically diverse structure in the foremost section of the intestinal canal , called the pharynx , in the polychaetes , the multi-bristled annelid worms . It is used for food intake. The Buccalorgan belongs to the ectodermal foregut , which means he is lined with a cuticle and does not serve even the digestion of food. The term was introduced by the zoologist and annelid expert Rodney Phillips Dales .

A buccal organ is missing in the little bristles and the leeches , within the polychaetes it is absent in groups with a filtering diet such as the families Sabellidae and Serpulidae . Instead, they developed a differently formed tentacle crown made up of palps.

In analogy to the defects in the Annelida also similar formations in the mouth or throat area in other strains of animals, such as the sometimes be Hakensaugwürmern (Monogenea) referred to as Buccalorgane.

construction

The buccal organ emerges from the primary mouth region of the annelid larva known as the stomodeum . In the adult worm, the corresponding section is called the pharynx (throat) if it has special muscles. In many groups, the pharynx can be turned outwards; it is then called proboscis ("trunk"). In some groups, structures inside are hardened (sclerotized), sometimes even further reinforced by mineral deposits, and thus form jaws. The diversity of the variously modified buccal organs, depending on the diet, is usually attributed to a few basic types.

In a number of groups, dorsolaterally (i.e. laterally in the upper part) there are two weakly muscular, lip-like folds which are densely covered with cilia and secrete a lot of mucus through glandular cells . These can continue forward to the underside of the prostomium . During the feeding process, the lips are everted out by the muscular pulling back of the surrounding front section; the animals use it to scrape off non-adherent organic matter from surfaces on which they feed. This structure is particularly typical for small, only a few millimeters long species that live on the substrate surface or in the system of gaps ( interstitial ) inside the substrate. Some authors do not consider this expression to be part of the buccal organ.

Other groups have an axial, non-muscular, evertable proboscis or proboscis. This structure is typical for substrate eaters who eat sand or mud and then digest the organic content it contains, such as many species of the Arenicolidae , Maldanidae , Capitellidae , Opheliidae , Orbiniidae and Paraonidae . The proboscis is everted via an increase in hydrostatic pressure and retracted again by retractor muscles. It is typically abundantly provided with mucous glands but hardly any cilia.

Very many groups have ventral (arranged on the abdominal side) buccal organs, which occur in a variety of types. The most common type is a ventral proboscis, which is mobile through dense, plate-like muscle groups with numerous transverse muscles. Often it is a pillow-like, muscular structure with which food substrate is either scraped or licked off the surface, or which is used to compact or stuff the food mass. Tooth-like or jaw-like thickenings can be formed for this purpose. There are all transitions from ventral buccal organs from short, pillow-like structures to long, protruding proboscis, which presumably have arisen convergent in different groups . This structure is typical of many representatives of the Scolecida . Depending on the fine structure and arrangement of the muscle fibers, different subtypes are distinguished.

A buccal organ consisting of an axial, muscular proboscis is typical of the Phyllodocida . It is enclosed in an equally muscled buccal cavity and can be extended with muscle power. The mouth opening is here at the tip of the proboscis. In some families there are also finger-like papillae ( Pisionidae , Hesionidae , Pilargidae and Syllidae ) that carry sensory cells. An evertable axillary proboscis is often combined with jaw formations, especially in the Eunicida , but also more tooth-like in the Glyceridae , Nereididae , Pisionidae and Polynoidae . The jaws are special formations of the epidermis and are formed by gland cells called gnathoblasts. They can be further strengthened by mineral deposits, but never contain chitin as in arthropods.

Literature and Sources

  • Gregory Rouse, Fredrik Pleijel: Polychaetes. Oxford University Press, 2001. ISBN 978-0-19-850608-9 . on pages 32 to 34.
  • Alexander Tzetlin & Günter Purschke (2005): Pharynx and intestine. In Thomas Bartolomaeus & Günther Purschke (editors): Morphology, Molecules, Evolution and Phylogeny in Polychaeta and Related Taxa. Hydrobiologia 535/536: 199-225.
  • Patricia A. Hutchings & Kristian Fauchald: Definition & General Description. In Christopher J. Glasby et al .: Class Polychaeta. Fauna of Australia. Volume 4a: Polychaetes and allies, the southern synthesis. Csiro Publishing, 2000. on page 5.