SCOLECIDA 245 



end of the protonephridium is closed by a so-called flame cell. 

 This is a large cell provided with long vibrating cilia which 

 project into the proximal end of the canal. The flat worms are 

 usually hermaphrodyte and the reproductive system is highly 

 complicated. 



535. Order i. The Turbellaria, or gliding worms, are usually small, 

 very much flattened, aquatic animals. The name refers to -the method 

 of locomotion which is effected through the cilia by which the surface of 

 the body is covered. The mouth is on the ventral side and is usually 

 provided with an eversible proboscis. The digestive tract is a simple 

 blind sack in the smallest microscopic forms, but in the larger ones it is 

 divided into three main trunks which have numerous branches. This 

 form of digestive tract is a substitute for a circulatory system. There are 

 usually 2-many simple eyes at the anterior end and over the brain. 



536. Order 2. The Trematoda are parasites and consequently show 

 more or less evidence of degeneration. In the form of the body they 

 resemble the Turbellaria, but the surface of the body is destitute of cilia 

 in the adult. The animal is provided with hold-fast organs in the form 

 of hooks and suckers. Commonly there are two suckers, one at the 

 anterior end enclosing the mouth and another further back on the ventral 

 side. The digestive tract is usually two forked but may be more com- 

 plexly branched. Eyes are only found in the ectoparasitic forms and 

 in some free-living larval stages of endoparasites. The life history of a 

 trematode is described in Part III. Page 366. 



537. Order 3. The Cestoda are all endoparasites and in the adult stage 

 are found only in the digestive tract of higher animals. Special sense 

 organs and digestive tract are both entirely wanting. The digested food 

 of the host is absorbed through the surface of the body. In place of a 

 head there is a hold-fast organ called a scolex which in the most common 

 forms has a circle of four suckers and sometimes also a circlet of hooks. 

 In a narrower region just below the scolex a process of strobilation takes 

 place by which the body of the parasite is formed. This is usually com- 

 posed of a long series of proglottides, the ones farthest from the scolex 

 being the oldest. The last segments may be "ripe" while new ones are 

 forming below the scolex. A pair of lateral nerves extend through the 

 body from segment to segment. There is also a pair of longitudinal 

 excretory tubes which are connected by transverse canals in each proglottis. 

 Each proglottis contains also a complete set of reproductive organs highly 



