Heterotrich Ciliates.

The term heterotrich as used here denotes ciliates which have different kinds of cilial structures on different parts of their bodies. In other systems, both Stentor and Stylonychia may be classed as hypotrichs. These variations highlight the difficulties involved in classifying organisms which may not be closely related to one another, but have developed very similar features and appearance by a process of convergent evolution.

  Stentor.

Stentor is one of the largest of the protozoa, and is common in most freshwater habitats. In addition to longitudinal rows (kineties) of fine cilia which cover the entire body ( see diagram), Stentor has, at its mouth end, a spiral arrangement of powerfully beating membranelles called cirri, each formed by the fusion of several cilia. The feeding current generated by these structures affects a large volume of water in their immediate vicinity, and bacteria, algae and other organisms caught up in the vortex are inevitably drawn into the buccal funnel where they are enclosed in food vacuoles, many of which can be seen circulating through the Stentor's cytoplasm at any given time.

A large, actively feeding Stentor creates currents powerful enough to capture passing rotifers and even small crustaceans such as young Daphnia. Stentor is the Charybdis of the protozoan world.

  Stentor:  A Detailed View.

The following pictures are of a partially contracted and free-swimming Stentor, and have been shot using electronic flash in darkfield illumination.
Magnification for all pictures is x400.
Picture captions read anticlockwise from the upper left.

	The Stentor swims slowly towards a free-swimming Coleps. This picture shows the cirri which surround the Stentor's buccal area. Frozen by the electronic flash, each of the cirri can be seen to be a membranelle made up of several cilia. Looking almost directly into the buccal cavity, this picture shows the full extent of the buccal area, with its spiral structure of cirri beginning to the left of the mouth itself, forming a circle which becomes the flange of the trumpet shape when the organism is fully extended, and descending in a tight spiral into the interior of the buccal funnel. Seen through the parallel rows (kineties) of fine cilia which cover the Stentor's exterior, the connected globular structure of the nucleus is clearly visible, stretching from left to right across the picture. Several food vacuoles are visible in this picture. The three which lie parallel to the upper surface contain Coleps in various states of disintegration. Just below the central vacuole, the digesting body of a Lecane rotifer is visible. The end of the buccal funnel can be faintly seen to the left. A clearer view of the body of the Lecane rotifer. Its foot extends towards the top of the picture. Here the Stentor is more fully extended, and the end of its body close to the point of attatchment is seen. The parallel lines of the kineties reveal the twisting of the outer pellicle.

  Other Heterotrichs.

Stylonychia can be considered a typical example of a heterotrich ciliate (although in some systems it is classed as a hypotrich) on account of the cirri on its ventral surface (underside) which act as legs, and the membranelle which follows the line of the oral groove (see diagram).

It is a common freshwater ciliate and has similar features to smaller heterotrichs such as Euplotes and Aspidisca, although genetic investigations involving ribosomal RNA and electron microscopy are beginning to reveal that many in this group are not so closely related, despite appearances.