Platyhelminthes

Nematoda

Annelida

Classification

Kingdom Animalia -> Phylum Patyhelminthes

(Flatworms)

three germ layers: flatworms (Dugesia, Planaria, Schistosoma). More than 18,000 species.

Classes: Turbellaria, Trematoda, Cestoda (tapeworms)

Kindgom Animalia -> Phylum Nematoda (Roundworms)

includes important human parasites such as Ascaris, pinworms, hookworms, Trichinella and Wuchereria: roundworms; more than 80,000 species

Kingdom Animalia -> Phylum Annelida

protostomes: annelids; about 15,000 species

Classes: Oligochaeta, Polychaeta, Hirudina

General Characteristics

Body flat and ribbon-like, without true segments; bilaterally symmetrical acoelomates; organs present;

Typically tiny, parasitic, unsegmented worms; body slender and enlongated;

Pseudocoelomates, meaning they have a pseudocoelom, a hollow fluid-filled cavity that is lined by mesoderm on the outside and endoderm on the inside (unlike the flatworm which does not have a hollow cavity between the germ layers).

Serially segmented worms: bilaterally symmetrical. Annelids have a segmented coelom. The different segments of the coelom allows asynchronous muscle contractions across the body. Most annelids have external bristles - setae - and fleshy protusions called parapodia.

Structure

Bilateral symmetry with ectoderm, endoderm, and mesoderm germ layers. Flatworms do not have a hollow cavity between the endoderm and mesoderm and thus are acoelomates. This structure allows for direct carbon-dioxide and oxygen exchange with the environment.

The structure of roundworms limits them to 1 mm to 120 cm (4 ft). Roundworms have a digestive tract with 2 openings as opposed to the flatworms' 1 opening. This means that food goes in one end and out another (or sometimes not, see the behavior observed in Chordata of throwing up).

“Most roundworms have separate sexes and are covered by a protective, noncellular layer called the cuticle.”

The structure has bristles across the surface. This is a protective/defensive structure. This suggests that at some point in the past (or still in the present) there was some type of predator that ate the worms. The members which developed the protective defensive measures (or perhaps they are offensive structures) were the ones which were “fit” and reproduced.

Response

. The flatworm is able to generate response due to neural control. The cerebral ganglia serves as a simple brain to process sensory signals. Light is also sensed through eyespots.

Unlike information on the flatworm the roundworms are not specifically mentioned to have neural information processing capacities. However, the existence of the two-exit digestive tract suggests that there is some type of behavior being controlled to find input (food) that is not the last waste from the other end of the digestive tract (food/substance differentiation).

The nervous system of an earthworm is a chain of ganglia. Ganglia are connected by a ventral nerve cord (evidence of centralization). Nerve cells are connected to muscle cells. Each segment has muscles and nerve cells in the same area. This allows independent response.

Reproduction

Planarians are hermaphroditic. When two planarians reproduce sexually they simultaneously fertilize each others opposite sex organs (testes/ovaries). Planarians reproduce asexually through fission. Planarians are known to be able to regenerate their bodies. In fact, one scientist, Thomas Morgan, discovered that a planarian was able to regenerate from a mere 10,000 cells off of the original body. This means that 1/300^th of the original organism was removed and it grew back into another organism.

The fluid-filled space is where eggs and sperm is stored with other organs. Muscles have developed to support the mechanism of reproduction and finding food (to attain energy for reproduction). Reproduction is not specifically mentioned in the text however is likely to be like the flatworm: asexual reproduction or fission.

Earthworms are also hermaphrodites. The worms have protective measures of fertilizing themselves. During the reproductive [mating] process mucus serves to keep the worms near to each other. The mucus is secreted by each worm's clitellum. On each other's bodies there are seminal receptors which filters the sperm from the mucus. A case develops out of chitin and about 2 to 3 weeks later a newly developing worm emerges.

Digestion

The process of digestion in the flatworm is a form of cephalization. Food is taken in and digested in the gastrovascular cavity.

pseudocoelomates with one-way gut; This implies that that there is a distinct entrance and a distinct exit. Furthermore there is the chance for digestive enzymes to be involved in the process. Unlike fungi the roundworms would digest internally, although like most Chordata some type of “saliva” (though specialization of that sort may not be present at this place in the tree of life).

Ingeniously, earthworms eat the soil that they borrow through. The earthworm eats soil which goes into the esophagus and then to a temporary storage area known as the crop. The muscular gizzard processes the soil, grinding it down. The gizzard passes the soil down the large intestine. During this process the digested organic compounds are absorbed by the blood. The typhlosole increases the surface area for absorption into the blood. Because earthworms digest soil they play an important role in farming and agriculture. There niche is important. Charles Darwin once estimated that a few hundred thousand earthworms per acre are in population, marveling the creatures.