Ciliates are protists (mostly single celled organisms), and among the largest, most active and structurally diverse of single celled organisms. Most are between 50 and 200 µm (1 µm = 1/1000 mm) in size. The pictures were taken with a microscope, and given that most ciliates are colorless, various devices are used to create contrast within the images.
Tetrahymena, upper left, is widely used as an experimental organism. This one has been stained to show the bases of the cilia, the large nucleus, and the mouth near the front of the cell. Cilia are densely packed around the mouth, and a channel that helps with the ingestion of food leads from the mouth into the cell. The second picture (Gastronauta) is of a living cell, and shows how the cilia are located in rows that make dotted lines from the anterior (top) to posterior. The angled line is a mouth. The third picture is of Paramecium cells that have been stained to show nuclei (they are red). There is a normal cell, two that are dividing horizontally – which is how they multiply – and a pair of cells that are fused near their mouths – they are conjugating, a process in which they exchange genetic material. Two cells enter the process and two cells leave it.
Ciliates have two kinds of nuclei – best seen in the fluorescence image of Paramecium bursaria (below), and in the stained Homalozoon.
Top right, Paramecium bursaria contains symbiotic algae that live and flourish within the cell. Below left is a cell that has been made to fluoresce, with abundant algae fluorescing red; and the (two types of) nuclei fluorescing green. To its right are three trumpet shaped Stentors. The broad ends of the cells each has a line of compound cilia that beat in a spanish wave that passes around the cell. To the left is another Stentor, this time with maroon pigmented granules and internal algae. Euplotes, like Stentor, has a curved lined of ciliary structures that beat to draw food towards the cell and into food vacuoles.
Peritrichs, such as Vaginicola and Vorticella, like Stentor, have rows of cilia around the tiop end of the cell, again they are used to make currents of water. The currents bring bacteria that they will eat to the mouth. Vaginicola lives in a small vase that provides some protection. Cells that stand up are prone to being eaten by small animals, and they (again like Stentor) get some protection by being able to contract. The image of Vorticella shows one contracted and one feeding cell. Alongside is a picture of a contracted stalk of a peritrich – with the contractile element inside.
In Bursaria we can see a channel that leads from the front of the cell towards the back. This is used to drive particules of the food into packages and into the cell for digestion.
The ciliates mentioned above feed on bacteria, most of the remainder are predators. Heliophrya and Tokophrya are suctoria. They have no cilia, but they sit still. Each radiating arm is a mouth, and if another ciliate swims into the arms, small packets of toxins and grappling irons in the expanded ends of the arms explode to kill and hold on to the food. Lacrymaria, Homalozoon and Loxophyllum slither around and if they encounter potential food, they will kill it with explosive barbs so it can be eaten. A picture shows Homalozoon stained so that we can see the nuclei. There is a very obvious string of red sausages – this is the macronucleus; and there are many small red dots, these being the micronuclei. Ciliates are one of the few types of organism to have more than one kind of nucleus. The macronucleus is very evident in Trithigmostoma. That ciliate also contains a basket of dark rods that it uses to prise bacteria off surfaces so it can eat them. Our final example of ciliates is Loxodes. Most live in lakes and ponds with little oxygen. They can ingest relatively large particles – such as diatoms.