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The Bdelloid Rotifers. 
Anatomy, using Philodina species.  

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Rotifers: Home Bdelloid Loricate Planktonic Sessile

  Bdelloid Rotifer Anatomy.

Bdelloid Rotifers: Intro Philodina


Philodina is one of the first rotifers to have been described in detail. Here is an early account of its feeding behaviour given by Henry Baker in a letter to the president of the Royal Society in 1745:
"A Couple of Circular Bodies, armed with small Teeth like those of the Balance-wheel of a Watch, appear projecting forwards beyond the Head, and extending sideways somewhat wider than the Diameter thereof. They have very much the Similitude of Wheels, and seem to turn round with a considerable Degree of Velocity, by which means a pretty rapid Current of Water is brought from a great distance to the very Mouth of the Creature, who is thereby supplied with very many little Animalcules and various Particles of Matter that the Waters are furnished with... "
It is also one of the most frequently encountered of rotifers. It is large and transparent with easily observed internal organs, and is not easily put off going about its normal business -- making it ideal for examination at high magnification.

Two Philodina rotifers Two extended Philodina rotifers with active coronas feed in a field traversed by filaments of algae. The yellow body is the loricate alga Trachelomonas, and a nearly developed egg of the rotifer Lecane is seen in the upper left of the picture.
Darkfield, 200X.
Philodina with algae A single Philodina from the same pond water sample as the pair above. It is viewed in ventral aspect, and the ciliated oesophagus can be seen tapering from the buccal cavity between the lobes of the corona to the trophi (jaws) and muscular bulb of the mastax (the surrounding muscular body which actuates the jaws).
Darkfield, 200X.
Philodina and algae A single Philodina browsing in a field of filamentous algae and flocculent organic matter.
Darkfield, 200X.
Philodina and algae Philodina feeding amongst filaments of algae.
Darkfield, 300X.
Philodina and algae Philodina feeding amongst filaments of algae.
Darkfield, 200X.
Philodina graveyard A rotifer graveyard. Philodina which die and sink to the bottom of the pond or specimen jar decay, leaving their jaws (trophi), the only solid part of their bodies, as their sole mortal remains.
This picture shows three sets of Philodina trophi in an amorphous organic mass containing numerous algal cells.
Darkfield, 1000X.
3 Philodina gregaria Three Philodina gregaria rotifers browse in a field of clumped organic matter. They are native to the Antarctic, and apart from the unusual bright red colouring of their bodies, are representative of bdelloid rotifers as a whole. Their anatomy is described in more detail in the group of micrographs below.
Rheinberg, 100X.

  Philodina gregaria.

Philodina gregaria is native to the Antarctic, where during the summer season, it can be found in such large numbers as to colour the bottoms of lakes and pools a rusty red. The well-known ability of rotifers, particularly the bdelloids, to survive long periods in the frozen state is clearly a survival necessity for P. gregaria, and the specimens in these pictures may be many years or even decades in age -- there is no way of telling.

The brightfield pictures were shot using a Leitz x100/1.2NA water immersion achromatic objective (designed for fluorescence work) and the low-power portion of a Watson Holos achromatic/aplanatic condenser at about 0.4NA. Electronic flash was used for all pictures.

Captions read anticlockwise from the upper left.

1. Philodina contracted

2. Philodina extended

3. Philodina feeding

4. Philodina. head, showing cilia.

  1. This specimen is contracted into the shape referred to as a tun (barrel shaped). Over a period of hours, as the temperature approaches freezing, a thick outer cuticle is developed, and excess water is expelled. Upon thawing, the process is reversed. Water is taken in, and within an hour or so, the rotifer is once again actively feeding. x200.
  2. An active Philodina in looping mode, with corona retracted. x200.
  3. A specimen extended and actively feeding, with its corona fully expanded. Between the lobes of the corona, the buccal cavity can be seen, with the ciliated oesophagus leading to the trophi and mastax. x200.
  4. This brightfield picture shows the metachronal waves of the coronal cilia frozen by electronic flash. The longer cilia of the upper lobes (the "wheels" of the wheel organ) contrast with the shorter cilia on the ventral projections forward of the buccal funnel. The oesophagus terminates at the trophi in the lower right corner of the picture.
    At this magnification (x1000) it is possible to appreciate the coordination of cilial activity which controls the feeding current bringing food into, and sending rejected particles out of, the buccal funnel.
  5. The trophi (jaws) are seen in the centre of the picture, with the lower section of the ciliated oesophagus immediately above. The two orange bodies at the top of the picture are the eyes. x1000.
  6. The lumen of the stomach. The more densely ciliated portions at the beginning and end are clearly seen.
    The lumen appears larger than in an active rotifer, as this specimen is compressed somewhat by the coverglass. x600.
  7. A flame cell (part of a primitive excretory organ called a protonephridium) can be seen enclosed in the circle in this picture. Philodina has two protonephridia, located at the sides of the head. Tubules connect the flame cells to a bladder which empties into the cloaca. x1500.
  8. A picture of an extended Philodina. The ciliated terminal section of the stomach lumen and the cloaca can be seen. A short anal canal is located in the body segment below the cloaca.
    The foot section begins at this point. x1000.
  9. The foot terminates in four small toes, each supplied with a sticky secretion from cement glands located higher up in the foot. In this picture, the toes are grouped tightly together, adhering to the underside of the coverglass. The two dorsal spurs are clearly seen. x1200.
5. Philodina: jaws and eyes 6. Philodina: stomach 7. Philodina: flame cell 8. Philodina: foot and cloaca 9. Philodina. toe with spurs.

More notes on rotifer anatomy are in preparation and will be uploaded soon. Until then, here is a link to some excellently concise External Website notes on the anatomy and behaviour of rotifers using Philodina as an example species.
... and another link to a first-hand account of a External Website dried rotifer cyst taking up water and resulting in an active rotifer (looks like a Philodina) resuming normal behaviour.