Metazoa

Most many-celled animals take solid food into some cavity within their bodies, in which it is dissolved or digested. This is not always the case, however. There are quite a few organisms which are able to digest their food outside of their bodies. Thus a starfish or sea urchin may pour digestive juices out over some fish or other large animal which it has captured. It is sometimes possible for an animal to eat other animals larger than itself. When a spider captures a fly, it pours digestive juices into the body of its prey and then sucks out the dissolved interior. Many parasitic animals require no digestive apparatus. In the case of animals such as tapeworms, there is no need for the ingestion of solid food, and the dissolved food substances in the alimentary canal of the host pass directly through the body wall of the animal. Other worms derive food in the same manner from the blood of their hosts.

Some authors have tried to classify animals on the basis of their methods of ingestion. Thus there have been distinctions between whirlers (ciliary feeders), snarers, scrapers and suckers. Small particles are often pulled into the body by ciliary action. This occurs not only in the protozoa, but also in practically every other phylum, except the arthropoda. Ciliary mechanisms for feeding are most highly developed in the mollusca. Tentacles and setae are also used in obtaining small particles. The animals that feed on large particles or masses have mechanisms for swallowing inactive food, for seizing prey, or for scraping and boring.

The usual form of ingestion in metazoa involves some sort of passage into the mouth. Jaws and teeth of various types may aid in the capture and in the subdivision of the food. The earthworm has a pharynx which is pulled on by muscles attached to its outer wall so that it can behave as a suction pump. Marine (polychaete) worms also may have sucking pumps. Thus in Autolytus the proventriculus, by its pulsations (120 per minute), produces a strong inward current of water and food. Sucking pumps also occur in insects of various orders, and these pumps may have a rather complicated structure. Even in the case of those animals which ingest food through the mouth there is sometimes a possibility that food may be taken in through the body wall.