CHAP. VIII 



PLA TYHELMINTHES 



95 



The only conspicuous structures to be seen on the upper sur- 

 face of the live creature, even with the aid of a strong lens, 

 are : the four groups of minute dark eye- 

 spots, between these a clear oval body 

 which is the brain, and, running longitu- 

 dinally down the body, an irregular, some- 

 times broken, streak ; this streak is the 

 central part of the digestive system, from 

 which many branches run out to the circum- 

 ference of the body. The cilia which cover 

 the body can only be seen under a high 

 magnifying power ; by their constant 

 movement they cause little whirlpools in 

 the surrounding water. Leptoplana is 

 carnivorous, its mouth is situated in 

 about the centre of the under side of the 

 body, and through it the large funnel-like 

 pharynx (throat) can be protruded, dis- 

 charging a digestive juice over the tissues 

 of the body of its prey. The creatures are 

 hermaphrodite but cross-fertilised. The 

 eggs are laid in the spring or summer, and 

 hatch in a few weeks. Leptoplana 

 is an example of the " poly clad " Tur- 

 bellarians, all of which are marine 

 forms with many lateral branches 

 to the stomach. 



Planaria ladea is an equally 

 common Turbellarian, but found in 

 fresh water ; it is a " triclad " Tur- 

 bellarian, i.e. one with only three 

 main branches to the stomach (see 

 Fig. 49). Like Leptoplana, it glides 

 along, but it may move also with 

 a looping motion, fixing the front end Fig. 49. — Planaria lactea. 

 of its body by means of a sucker on iP'^r""^^' ''^I'T^'"", 



,, , r r 1 1 T T the Cambridge Natural 



the lower surface of the head, and History, vol. ii.) 



then drawing the body up behind, p^, pharynx or guuet ; mo, mouth. 



It is about I of an inch long. 



Mesodoma ehrenhergii, another leaf - like Turbellarian 

 common in ponds, is a type of those with a straight, rod-like 



Fig. 48. — leptoplana 

 tremellaris x 3. 



( Upper surface. ) 



