Feeding Mechanism in Bryozoa

Bryozoans are sessile, aquatic, colonial coelomates. They are ciliary feeders, apparently without the assistance of mucus. The feeding apparatus of bryozoa plays an important role in their feeding. A current of water, made by the lateral ciliary tracts of tentacles, containing nutritive particles moves down the center of the tentacular funnel towards the mouth and out between the tentacles. The ciliary beats can be stopped or reversed it’s speed allied, perhaps as a means of preventing the escape of food particles in the outgoing current.

1. Protrusion of lophophore:-

   1. During feeding, the lophophore expands into a funnel shaped structure.

   2. In the proximal part contraction of 2 bands of transverse parietal muscles pull the ventral and dorsal walls of the flexible zoecium close together.

   3. This contraction reduces the coelomic volume and raises the internal hydrostatic pressure.

   4. The contraction of transverse muscles is followed by the relaxation of longitudinal, parietal and retractor muscles.

   5. The contraction of transverse parietal muscle raises the pressure still further.

   6. Further relaxation of the retractor muscles permit the tentacles to raise through the collar.

   7. Then the tentacles are spread by the action of their intrinsic musculature.

2. Production of feeding current :-
   

   When the lophophore is protruded, the lateral ciliated tracts on the tentacles create a current that sweeps downward into a funnel and passes outward and between tentacles. Regardless of the tentacle length and numbers, the distance between adjacent tentacle tips are generally about 110 small phytoplanktonic organisms which are probably the principle food of bryozoans, are driven into the funnel with the water current and on touching the lateral cilia, effect a local reversal beat which bounds the particles back into the upstream side of the tentacles and down towards the mouth, this is thus a ciliary upstream collecting system.

3. Hypothesis with regard to the feeding current in Bryozoa:-
   
   1. Impingement feeding hypothesis.
   2. Ciliary reversal hypothesis.
   3. Rejection of unwanted particles.
   4. Ingestion

4. Withdrawal of lophophore:- Withdrawal is achieved by the extremely rapid contraction of the retractor muscles while the transverse parietal muscles relaxed.

5. Passage of food through the Alimentary tract:- Frequent inward flicking of the lophophores help the food particles to go into the alimentary canal. The entry part of the stomach is a muscular gizzard lined with chitinous teeth that help to crush hard shelled prey. Tentacles flick to help to move the food through the stomach for digestion. The final part of the stomach is lined with microvilli looked cilia or minute hairs that compress undigested solids, which then pass through the intestine and out through the anus.