Baikal

Zoological excursions

Flatworms (Plathelminthes)

In Lake Baikal, there are more than 140 species of endemic flatworms – planarians. On the ventral side, there is a mouth with a protruding pharynx. A branchy intestine is seen through transparent covers of the body. These worms are diverse in the coastal zone of Lake Baikal. Their bodies are of bright colouration with patchy drawing. Those worms living under stones and at the water edge are 0.5-2 cm long, while at depths they can reach 30 cm. These worms moving around the bottom of the lake find a prey, envelop it with mucus and slowly pull inside the body. Feeding on sick and weak animals, planarians play the role of purifiers.

Another type of flatworms – Baicalobia variegate – inhabits the surf zone and deeper areas. This large worm is of oval shape 20 mm long and 7-8 mm wide. In the middle of the dark back there is a double dark stripe and two longitudinal rows of yellow-brown spots. Eyes are located on the sides of the anterior end of the body in a form of a horseshoe, each of which having 12-15 eyelets. Another species of planarians inhabiting the same areas is Bdellоcephala angarensis. This is a larger planarian (50 mm long and 10-15 mm wide). The body becomes wider at contraction. There are two large eyes on the head near the internal side of light spots. The colour of the back part of the body is brownish with irregular dark-brown spots. The peculiar feature of this planarian is that it forms wavy folds while moving.

Baicalobia variegata	Bdellоcephala angarensis
Rhimacephalus pulvinar

A planarian Rhimacephalus pulvinar inhabits silt and sandy soils at depths from 8-10 m to 100 m and deeper. It may be 110 mm long. The colouration of this planarian is light-gray or even dull yellow with longitudinal rows of irregular brown or almost black spots. In the middle of the back there is a dark stripe. There are 20-25 eyes on each side.