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Aquatic Invertebrates



Flatworm of the genus Dugesia.
A flatworm of the genus Dugesia. Its pharynx is extended out its body to catch prey. Two eyes on the animal's head are visible in the upper left corner of the photo.

Class: Turbellaria - Turbellarians (Flatworms)
Order: Tricladida - Triclads
Family: Planariidae - Planarians

Characteristics of the family:
These worms are elongated and flattened, much like leeches. Flatworms are both predators and scavengers. The mouth is located on the bottom or underneath surface. Almost all species are hermaphroditic, which means they have both male and female sexual organs.

Interesting fact:
Flatworms can also reproduce by splitting in half (fission). The missing body parts regenerate. Because of this peculiarity, flatworms in this family are commonly studied in laboratories.

Representative genus: Dugesia sp.
Description Elongated, flattened and tapered at both ends with eye spots (these are true but crude eyes) on the head. Range of colouration is of gray, brown, black and patterns of spots, stripes or mottling. Single opening on ventral surface that acts as both mouth and anus.
Size 5 to 30 mm
Distribution Found mostly east of the Great Plains.
Habitat Flatworms in this class are photonegative (they avoid light) and are found under rocks and debris during the day. Unlike most flatworms, those in this class live in freshwater aquatic systems such as ponds, lakes, ditches, rivers and streams.
Food Flatworms are predators and scavengers. Their mouth is located ventrally (on their belly). The pharynx is extruded to swallow whole or bite a chunk of their prey, which includes snails, worms or soft insect larvae.
Life cycle Can reproduce asexually by fission, in which the animal splits in half and then regenerates the missing body parts. Sexual reproduction between a pair of hermaphroditic flatworms results in a few eggs being laid in a cocoon.


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A Project of the Canadian Museum of Nature
 Images: Ed Hendrycks