flatworm - Body functions, Classes

A flattened, worm-like animal with a definite head but without a true body cavity (coelom); digestive system usually lacks an anus; free living flatworms typically feed on small invertebrates; parasitic forms include tapeworms and flukes. (Phylum: Platyhelminthes.)

iFlatworms
"Platodes" from Ernst Haeckel's Kunstformen der Natur, 1909
Scientific classification
Kingdom: Animalia Subkingdom: Eumetazoa Superphylum: Platyzoa Phylum: Platyhelminthes Gegenbaur, 1859
Classes
Monogenea Trematoda Cestoda Turbellaria

The flatworms (Platyhelminthes, Greek "platy"': flat; Most flatworms are free-living forms, but many are parasitic on other animals. Flatworms are the simplest triploblastic animals with organs.

Body functions

There is also no true body cavity (coelom) except the gut and hence they are acoelomates. There is no true circulatory or respiratory system, but like all other animals, flatworms do take in oxygen.

Flatworms respire at their integument;

However, flatworms do have a bilateral nervous system;

Usually the digestive tract has one opening, so the animal can't feed, digest, and eliminate undigested particles of food simultaneously, as most animals with tubular guts can. However, in a few particularly long flatworms or those with highly branched guts, there may be one or more anuses. A small group where the gut is absent or non-permanent, called acoel flatworms, appear to be unrelated to the other Platyhelminthes (see below).

Muscular contraction in the upper end of the gut causes a strong sucking force allowing flatworms to ingest their food and tear it into small bits.

Flatworm reproduction is hermaphroditic, meaning each individual produces eggs and sperm. Each half grows replacements of the missing pieces to form two whole flatworms. This also means that if one of these flatworms is cut in half, each half will regenerate into two separate fully-functioning flatworms.

Classes

Flatworms were formerly considered to be basal among the protostomes. The true flatworms form a monophyletic group that developed from more complex ancestors, and grouped with several other phyla as the Platyzoa. The traditional classifications of flatworms is primarily based on differing degrees of parasitism and divided into three monophyletic classes:

Trematoda - Flukes, probably paraphyletic to Cestoda.

The remaining flatworms are grouped together for convenience as the class Turbellaria, now comprising the following orders:

Catenulida Macrostomida Lecithoepitheliata Rhabdocoela Prolecithophora Proseriata Tricladida Polycladida

Most of these groups include free-living forms.