As described by Webster 1913, the word "sessile" comes from the Latin for "to sit" and again, as described above, is used in two senses in biology. The zoological definition is, I think, more interesting.
One of the most basic facts about animal life is that animals move about. This is one of the simplest facts in biology, presented at even the kindergarten level as a way to classify living things into animals and plants. Despite the fact that cladistics has obviously gotten far more complicated, this is still probably the rule of thumb that most people use for deciding what is or is not an animal. Like most simple rules, there are many and varied exceptions to the rule. The exceptions to the rule are called sessile animals, literally "sitting" animals. The Webster 1913 example points out an example from Cnidaria, one of the simplest phylums of animal, but every major phylum of animal has some members who have a sessile lifestyle.
These are some examples of sessile animals, by phylum:

  • Bryozoa, a group of small, colonial filter feeders are all sessile, as their name "moss animals" would suggest.
  • Porifera, or the sponges are another all-sessile group of filter feeders.
  • Cnidaria, which includes the jellies and sea anemones, many of which are sessile for part or all of their life.
  • Mollusks, which are usually more sophisticated than the above mentioned groups, also include many sessile members amongst their free-living members. These include the many clams and mussels..
  • Annelid worms have several groups, such as the polycheate worms, that live rooted in one spot. Other groups of worm like creatures, such as the priapulid worms, that are not closely related to worms but share their body plan, also are stationary.
  • Arthropods have one major group of sessile creatures: the Crustacean barnacles.
  • Echinoderm, which are more closely related to the Chordata despite their primative nervous systems, have the Crinoids, another group of filter feeders.
  • Chordata, our own phylum, have one small group of sessile creatures. Although there are no sessile vertebrates, there are creatures with a nervous chord that, after an earlier larval stage, root themselves in one place. These are the tuniculates or sea squirts.

This brief list (which again, only covers the major phylum) shows that the sessile lifestyle comes up again and again. Some of the earliest families of animals seem to have been exclusively sessile, and it could be that mobile animal life developed from larval forms, which, through a process of neotony, reached maturity without finding a place to attach to the ground. Or it could be that the sessile form was returned to over the ages separately. I beg my own ignorance on the matter, as well as probably the ignorance of science. Most of the fossil evidence for small, soft bodied creatures is necessarily scant.
Becoming or staying sessile does make some sense, in certain circumstances. Constantly moving around requires a lot of energy, so by sitting still in one place, an animal automatically reduces their need for food intake. Since all sessile animals also relatively simple body plans, and low metabolic needs, this means they need very little food, one of the reasons why you can find a rock totally covered with barnacles. Also, because sessile animals don't need to move, there is nothing preventing them from growing thick armor, as many do. However, the sessile form has some draw backs, the largest one being rather obvious. Sitting in one place, a sessile animal has to wait for its food to come to it. That is why all of the sessile animals listed above are aquatic, and almost all of them are filter feeders, with the exception of a few predators such as the sea anemone. When you are waiting for food to come to you, you can't be picky about what you get. Being filter feeders means that a sessile animal would never have the ability to have a fast metabolism or complicated body plan (or nervous system), and that they are thus (pardon the pun) stuck in one place.
So while becoming sessile seems to be a viable option, and one that animals return to repeatedly, it also has some obvious limits. What is valuable in observing the sessile animals is that it shows that just as with any scheme of classification, there can be large and important exceptions to our rules about what a class is like.

Ses"sile (?), a. [L. sessilis low, dwarf, from sedere, sessum, to sit: cf. F. sessile.]

1.

Attached without any sensible projecting support.

2. Bot.

Resting directly upon the main stem or branch, without a petiole or footstalk; as, a sessile leaf or blossom.

3. Zool.

Permanently attached; -- said of the gonophores of certain hydroids which never became detached.

 

© Webster 1913.

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